diff --git a/ttcn3/syntax/nodes.go b/ttcn3/syntax/nodes.go
index 6c46188d..c66aaf49 100644
--- a/ttcn3/syntax/nodes.go
+++ b/ttcn3/syntax/nodes.go
@@ -576,11 +576,12 @@ type (
 
 	// A BehaviourSpec represents a behaviour type specification.
 	BehaviourSpec struct {
-		KindTok Token       // TESTCASE, FUNCTION, ALTSTEP
-		Params  *FormalPars // Parameter list or nil
-		RunsOn  *RunsOnSpec // runs on spec or nil
-		System  *SystemSpec // system spec or nil
-		Return  *ReturnSpec // return value spec or nil
+		KindTok    Token       // TESTCASE, FUNCTION, ALTSTEP
+		Interleave Token       // INTERLEAVE or nil
+		Params     *FormalPars // Parameter list or nil
+		RunsOn     *RunsOnSpec // runs on spec or nil
+		System     *SystemSpec // system spec or nil
+		Return     *ReturnSpec // return value spec or nil
 	}
 )
 
@@ -636,18 +637,19 @@ type (
 
 	// A FuncDecl represents a behaviour definition.
 	FuncDecl struct {
-		External Token // Position of "external" or nil
-		KindTok  Token // TESTCASE, ALTSTEP, FUNCTION
-		Name     *Ident
-		Modif    Token // Position of "@deterministic" or nil
-		TypePars *FormalPars
-		Params   *FormalPars // Formal parameter list or nil
-		RunsOn   *RunsOnSpec // Optional runs-on-spec
-		Mtc      *MtcSpec    // Optional mtc-spec
-		System   *SystemSpec // Optional system-spec
-		Return   *ReturnSpec // Optional return-spec
-		Body     *BlockStmt  // Body or nil
-		With     *WithSpec
+		External   Token // Position of "external" or nil
+		KindTok    Token // TESTCASE, ALTSTEP, FUNCTION
+		Interleave Token // INTERLEAVE or nil
+		Name       *Ident
+		Modif      Token // Position of "@deterministic" or nil
+		TypePars   *FormalPars
+		Params     *FormalPars // Formal parameter list or nil
+		RunsOn     *RunsOnSpec // Optional runs-on-spec
+		Mtc        *MtcSpec    // Optional mtc-spec
+		System     *SystemSpec // Optional system-spec
+		Return     *ReturnSpec // Optional return-spec
+		Body       *BlockStmt  // Body or nil
+		With       *WithSpec
 	}
 
 	// A ConstructorDecl represents a class constructor definition.
@@ -728,15 +730,16 @@ type (
 
 	// A BehaviourTypeDecl represents a named behaviour type.
 	BehaviourTypeDecl struct {
-		TypeTok  Token // Position of "type"
-		KindTok  Token // TESTCASE, ALTSTEP, FUNCTION
-		Name     *Ident
-		TypePars *FormalPars
-		Params   *FormalPars // Formal parameter list
-		RunsOn   *RunsOnSpec // Optional runs-on spec
-		System   *SystemSpec // Optional system spec
-		Return   *ReturnSpec // Optional return spec
-		With     *WithSpec
+		TypeTok    Token // Position of "type"
+		KindTok    Token // TESTCASE, ALTSTEP, FUNCTION
+		Interleave Token // INTERLEAVE or nil
+		Name       *Ident
+		TypePars   *FormalPars
+		Params     *FormalPars // Formal parameter list
+		RunsOn     *RunsOnSpec // Optional runs-on spec
+		System     *SystemSpec // Optional system spec
+		Return     *ReturnSpec // Optional return spec
+		With       *WithSpec
 	}
 
 	PortTypeDecl struct {
diff --git a/ttcn3/syntax/nodes_gen.go b/ttcn3/syntax/nodes_gen.go
index 077b73fd..e01182af 100644
--- a/ttcn3/syntax/nodes_gen.go
+++ b/ttcn3/syntax/nodes_gen.go
@@ -93,6 +93,9 @@ func (n *BehaviourSpec) FirstTok() Token {
 	case n.KindTok != nil:
 		return n.KindTok
 
+	case n.Interleave != nil:
+		return n.Interleave
+
 	case n.Params != nil:
 		return n.Params.FirstTok()
 
@@ -125,6 +128,9 @@ func (n *BehaviourSpec) LastTok() Token {
 	case n.Params != nil:
 		return n.Params.LastTok()
 
+	case n.Interleave != nil:
+		return n.Interleave
+
 	case n.KindTok != nil:
 		return n.KindTok
 
@@ -134,12 +140,16 @@ func (n *BehaviourSpec) LastTok() Token {
 }
 
 func (n *BehaviourSpec) Children() []Node {
-	ret := make([]Node, 0, 5)
+	ret := make([]Node, 0, 6)
 
 	if n.KindTok != nil {
 		ret = append(ret, n.KindTok)
 	}
 
+	if n.Interleave != nil {
+		ret = append(ret, n.Interleave)
+	}
+
 	if n.Params != nil {
 		ret = append(ret, n.Params)
 	}
@@ -218,6 +228,9 @@ func (n *BehaviourTypeDecl) FirstTok() Token {
 	case n.KindTok != nil:
 		return n.KindTok
 
+	case n.Interleave != nil:
+		return n.Interleave
+
 	case n.Name != nil:
 		return n.Name.FirstTok()
 
@@ -268,6 +281,9 @@ func (n *BehaviourTypeDecl) LastTok() Token {
 	case n.Name != nil:
 		return n.Name.LastTok()
 
+	case n.Interleave != nil:
+		return n.Interleave
+
 	case n.KindTok != nil:
 		return n.KindTok
 
@@ -280,7 +296,7 @@ func (n *BehaviourTypeDecl) LastTok() Token {
 }
 
 func (n *BehaviourTypeDecl) Children() []Node {
-	ret := make([]Node, 0, 9)
+	ret := make([]Node, 0, 10)
 
 	if n.TypeTok != nil {
 		ret = append(ret, n.TypeTok)
@@ -290,6 +306,10 @@ func (n *BehaviourTypeDecl) Children() []Node {
 		ret = append(ret, n.KindTok)
 	}
 
+	if n.Interleave != nil {
+		ret = append(ret, n.Interleave)
+	}
+
 	if n.Name != nil {
 		ret = append(ret, n.Name)
 	}
@@ -3545,6 +3565,9 @@ func (n *FuncDecl) FirstTok() Token {
 	case n.KindTok != nil:
 		return n.KindTok
 
+	case n.Interleave != nil:
+		return n.Interleave
+
 	case n.Name != nil:
 		return n.Name.FirstTok()
 
@@ -3613,6 +3636,9 @@ func (n *FuncDecl) LastTok() Token {
 	case n.Name != nil:
 		return n.Name.LastTok()
 
+	case n.Interleave != nil:
+		return n.Interleave
+
 	case n.KindTok != nil:
 		return n.KindTok
 
@@ -3625,7 +3651,7 @@ func (n *FuncDecl) LastTok() Token {
 }
 
 func (n *FuncDecl) Children() []Node {
-	ret := make([]Node, 0, 12)
+	ret := make([]Node, 0, 13)
 
 	if n.External != nil {
 		ret = append(ret, n.External)
@@ -3635,6 +3661,10 @@ func (n *FuncDecl) Children() []Node {
 		ret = append(ret, n.KindTok)
 	}
 
+	if n.Interleave != nil {
+		ret = append(ret, n.Interleave)
+	}
+
 	if n.Name != nil {
 		ret = append(ret, n.Name)
 	}
diff --git a/ttcn3/syntax/parser.go b/ttcn3/syntax/parser.go
index 73e02289..a1b68538 100644
--- a/ttcn3/syntax/parser.go
+++ b/ttcn3/syntax/parser.go
@@ -1833,6 +1833,11 @@ func (p *parser) parseBehaviourTypeDecl() *BehaviourTypeDecl {
 	x := new(BehaviourTypeDecl)
 	x.TypeTok = p.consume()
 	x.KindTok = p.consume()
+
+	if p.tok == INTERLEAVE {
+		x.Interleave = p.consume()
+	}
+
 	x.Name = p.parseName()
 	if p.tok == LT {
 		x.TypePars = p.parseTypeFormalPars()
@@ -2001,6 +2006,11 @@ func (p *parser) parseBehaviourSpec() *BehaviourSpec {
 
 	x := new(BehaviourSpec)
 	x.KindTok = p.consume()
+
+	if p.tok == INTERLEAVE {
+		x.Interleave = p.consume()
+	}
+
 	x.Params = p.parseFormalPars()
 
 	if p.tok == RUNS {
@@ -2177,6 +2187,10 @@ func (p *parser) parseFuncDecl() *FuncDecl {
 
 	x := new(FuncDecl)
 	x.KindTok = p.consume()
+
+	if p.tok == INTERLEAVE {
+		x.Interleave = p.consume()
+	}
 	if p.tok == MODIF {
 		x.Modif = p.consume()
 	}
diff --git a/ttcn3/syntax/parser_test.go b/ttcn3/syntax/parser_test.go
index 320652b9..238b74a7 100644
--- a/ttcn3/syntax/parser_test.go
+++ b/ttcn3/syntax/parser_test.go
@@ -105,6 +105,7 @@ func TestFuncDecls(t *testing.T) {
 		{pass, `function f() mtc C {}`},
 		{pass, `function f() runs on C mtc C system C {}`},
 		{pass, `altstep as() { var roi[-] a[4][4]; [] receive; [else] {}}`},
+		{pass, `altstep interleave interleaveStep() {  [] p1.receive; [] p2.check{}}`},
 		{pass, `signature f();`},
 		{pass, `signature f() exception (integer);`},
 		{pass, `signature f() return int;`},
@@ -235,6 +236,8 @@ func TestTypes(t *testing.T) {
 		{pass, `type set s {int a optional }`},
 		{pass, `type set s {set length(1) of set length(2) of int() f1[-][-] length(3) optional}`},
 		{pass, `type set s {function () runs on self return template int callback}`},
+		{pass, `type set s {altstep (integer p_i) runs on self as_callback}`},
+		{pass, `type set s {altstep interleave(integer p_i) runs on C is_callback}`},
 		{pass, `type union s {@default set of int f1 optional}`},
 		{pass, `type union s {enumerated { e(1) } foo}`},
 		{pass, `type enumerated a {e, e(1), e(1)}`},
@@ -260,6 +263,7 @@ func TestTypes(t *testing.T) {
 		// Behaviour Types
 		{pass, `type function fn() runs on self return template int`},
 		{pass, `type altstep  as() runs on self return int`},
+		{pass, `type altstep interleave iStep() runs on C`},
 		{pass, `type testcase tc() runs on C system TSI`},
 
 		// Class Types