diff --git a/argcomplete/bash_completion.d/_python-argcomplete b/argcomplete/bash_completion.d/_python-argcomplete
index b29d9cd..3023ff2 100644
--- a/argcomplete/bash_completion.d/_python-argcomplete
+++ b/argcomplete/bash_completion.d/_python-argcomplete
@@ -244,7 +244,17 @@ else
     # -Uz is recommended for the use of functions supplied with the zsh distribution.
     # https://unix.stackexchange.com/a/214306
     autoload -Uz is-at-least
-    # The comment at the top of this file causes zsh to invoke this script directly,
+    # If this is being implicitly loaded because we placed it on fpath,
+    # the comment at the top of this file causes zsh to invoke this script directly,
     # so we must explicitly call the global completion function.
-    _python_argcomplete_global
+    # Note $service should only ever be -default- because the comment at the top
+    # registers this script as the default completer (#compdef -default-).
+    if [[ $service == -default- ]]; then
+        _python_argcomplete_global
+    fi
+    # If this has been executed directly (e.g. `eval "$(activate-global-python-argcomplete --dest=-)"`)
+    # we need to explicitly call compdef to register the completion function.
+    # If we have been implicitly loaded, we still call compdef as a slight optimisation
+    # (there is no need to execute any top-level code more than once).
+    compdef _python_argcomplete_global -default-
 fi
diff --git a/test/test.py b/test/test.py
index bae9ece..c8f9419 100755
--- a/test/test.py
+++ b/test/test.py
@@ -1409,11 +1409,13 @@ class TestBashGlobal(TestBash, TestBashZshGlobalBase):
     pass
 
 
-class TestZshGlobal(TestZsh, TestBashZshGlobalBase):
-    # In zsh, the file is not sourced directly;
+class TestZshGlobalExplicit(TestZsh, TestBashZshGlobalBase):
+    pass
+
+
+class TestZshGlobalImplicit(TestZsh, TestBashZshGlobalBase):
+    # In zsh, the file is typically not sourced directly;
     # it is added to fpath and autoloaded by the completion system.
-    # Running `eval "$(activate-global-python-argcomplete --dest=-)"`
-    # as in bash does *not* work in zsh!
     zsh_fpath = os.path.join(os.path.abspath(os.path.dirname(argcomplete.__file__)), "bash_completion.d")
     init_cmd = f'fpath=( {zsh_fpath} "${{fpath[@]}}" ); autoload compinit; compinit -u'
     install_cmd = None