diff --git a/src/workerd/io/observer.h b/src/workerd/io/observer.h
index 9b439fd6a49..2af08161985 100644
--- a/src/workerd/io/observer.h
+++ b/src/workerd/io/observer.h
@@ -74,6 +74,9 @@ class RequestObserver: public kj::Refcounted {
 
 class IsolateObserver: public kj::AtomicRefcounted {
 public:
+  virtual void created(kj::StringPtr id) {};
+  // Called when Worker::Isolate is created.
+
   virtual void evicted() {}
   // Called when the owning Worker::Script is being destroyed. The IsolateObserver may
   // live a while longer to handle deferred proxy requests.
diff --git a/src/workerd/io/worker.c++ b/src/workerd/io/worker.c++
index 054469fc550..389de29723d 100644
--- a/src/workerd/io/worker.c++
+++ b/src/workerd/io/worker.c++
@@ -977,6 +977,7 @@ Worker::Isolate::Isolate(kj::Own<ApiIsolate> apiIsolateParam,
       metrics(kj::mv(metricsParam)),
       impl(kj::heap<Impl>(*apiIsolate, *metrics, *limitEnforcer, allowInspector)),
       weakIsolateRef(kj::atomicRefcounted<WeakIsolateRef>(this)) {
+  metrics->created(id);
   // We just created our isolate, so we don't need to use Isolate::Impl::Lock (nor an async lock).
   auto lock = apiIsolate->lock();
   auto features = apiIsolate->getFeatureFlags();