diff --git a/inc/SimInfo.hh b/inc/SimInfo.hh
index 81af4f1..dd2b276 100644
--- a/inc/SimInfo.hh
+++ b/inc/SimInfo.hh
@@ -29,6 +29,7 @@ namespace mu2e
     XYZVectorF endpos = XYZVectorF();  // end position of the SimParticle [mm, in detector coords]
     MCRelationship prirel = MCRelationship(); // relationship to the event primary particles
     MCRelationship trkrel = MCRelationship(); // relationship to the particle that created hits in the track
+    std::vector<int> ancestorSimIds; // full parent chain: [parentId, grandparentId, ..., rootId]
     void reset() { *this = SimInfo(); }
   };
 }
diff --git a/src/InfoMCStructHelper.cc b/src/InfoMCStructHelper.cc
index 61fb2e5..5bae4c0 100644
--- a/src/InfoMCStructHelper.cc
+++ b/src/InfoMCStructHelper.cc
@@ -289,6 +289,15 @@ namespace mu2e {
     siminfo.pos = XYZVectorF(det->toDetector(sp.startPosition()));
     siminfo.endpos = XYZVectorF(det->toDetector(sp.endPosition()));
     siminfo.endmom = XYZVectorF(sp.endMomentum());
+    // Walk the Geant4 parent chain and store every ancestor's SimParticle id
+    // (immediate parent first, root last). Populated here so every SimInfo
+    // collection (trkmcsim, calomcsim, primary list) exposes the same
+    // ancestorSimIds branch consistently.
+    auto p = sp.parent();
+    while (p.isNonnull()) {
+      siminfo.ancestorSimIds.push_back(p->id().asInt());
+      p = p->parent();
+    }
   }
 
   void InfoMCStructHelper::fillVDInfo(const KalSeed& kseed, const KalSeedMC& kseedmc, std::vector<std::vector<MCStepInfo>>& all_vdinfos) {