diff --git a/src/plantuml_gui/add.py b/src/plantuml_gui/add.py
index 36b83b1..a7753d5 100644
--- a/src/plantuml_gui/add.py
+++ b/src/plantuml_gui/add.py
@@ -23,6 +23,7 @@
 # SOFTWARE.
 
 
+import re
 from typing import Literal
 
 
@@ -46,7 +47,18 @@ def add(
     lines = puml.splitlines()
 
     if type == "activity":
-        lines.insert(index, ":Activity;")
+        activity_numbers = []
+        for line in lines:
+            match = re.search(r":Activity (\d+)", line)
+            if match:
+                activity_numbers.append(int(match.group(1)))
+
+        if activity_numbers:
+            next_activity_number = max(activity_numbers) + 1
+        else:
+            next_activity_number = 1
+
+        lines.insert(index, f":Activity {next_activity_number};")
 
     if type == "connector":
         lines.insert(index, "(C)")
diff --git a/tests/test_app.py b/tests/test_app.py
index 9dc9c3d..0756843 100644
--- a/tests/test_app.py
+++ b/tests/test_app.py
@@ -286,7 +286,7 @@ def test_addtomergenoifmerge3(self, client):
 endif
 detach
 repeat
-:Activity;
+:Activity 1;
 :Activity;
 backward:Activity;
 repeat while (while ?) is (yes) not (no)
@@ -329,7 +329,7 @@ def test_addtomergenoifmerge2(self, client):
 detach
 endif
 repeat
-:Activity;
+:Activity 1;
   :Activity;
   backward:Activity;
 repeat while (while ?) is (yes) not (no)
@@ -372,7 +372,7 @@ def test_addtomergenoifmerge(self, client):
   :Activity;
 endif
 repeat
-:Activity;
+:Activity 1;
   :Activity;
   backward:Activity;
 repeat while (while ?) is (yes) not (no)
@@ -447,7 +447,7 @@ def test_addtomerge2(self, client):
 else (no)
   :Activity;
 endif
-:Activity;
+:Activity 1;
 repeat
   :Activity;
   :Activity;
@@ -519,7 +519,7 @@ def test_addactivitymerge(self, client):
 else (no)
   :Actaivity;
 endif
-:Activity;
+:Activity 1;
 :Activity;
 endif
 :Activity;
@@ -1743,7 +1743,7 @@ def test_add_activity_connector2(self, client):
 note
 end note
 detach
-:Activity;
+:Activity 1;
 @endumll"""
             assert response.data.decode("utf-8") == expected_puml
 
@@ -1950,7 +1950,7 @@ def test_addactivitybelowwithconnector(self, client):
 (A)
 detach
 stop
-:Activity;
+:Activity 1;
 @enduml"""
             assert response.data.decode("utf-8") == expected_puml
 
@@ -3765,7 +3765,7 @@ def test_addtofork(self, client):
 fork again
   :action;
 end fork
-:Activity;
+:Activity 1;
 @enduml"""
             assert response.data.decode("utf-8") == expected_puml
 
@@ -4377,6 +4377,27 @@ def test_addtoactivity(self, client):
 backward:Activity;
 repeat while (while ?) is (yes) not (no)
 :Activity;
+@enduml"""
+            assert response.data.decode("utf-8") == expected_puml
+
+    def test_addactivitytoactivity(self, client):
+        test_data = {
+            "plantuml": """@startuml
+:Activity 1;
+@enduml""",
+            "svg": """<rect fill="#F1F1F1" height="33.9688" rx="12.5" ry="12.5" style="stroke:#181818;stroke-width:0.5;" width="75" x="11" y="11"></rect><text fill="#000000" font-family="sans-serif" font-size="12" lengthAdjust="spacing" textLength="55" x="21" y="31.9688" style="pointer-events: none;">activity 1</text>""",
+            "svgelement": """<rect fill="#F1F1F1" height="33.9688" rx="12.5" ry="12.5" style="stroke:#181818;stroke-width:0.5;" width="75" x="11" y="11"></rect>""",
+            "type": "activity",
+        }
+        with client:
+            response = client.post(
+                "/addToActivity",
+                data=json.dumps(test_data),
+                content_type="application/json",
+            )
+            expected_puml = """@startuml
+:Activity 1;
+:Activity 2;
 @enduml"""
             assert response.data.decode("utf-8") == expected_puml