diff --git a/src/particle.cpp b/src/particle.cpp
index 3a05d11ff5..b6e936b620 100644
--- a/src/particle.cpp
+++ b/src/particle.cpp
@@ -420,6 +420,7 @@ Particle::cross_surface()
     site.E = this->E_;
     site.wgt = this->wgt_;
     site.delayed_group = this->delayed_group_;
+    site.surf_num = surf->id_;
     site.particle = this->type_;
     // site.parent_id = this->id_;
     // site.progeny_id = this->n_progeny_;