diff --git a/cromulant/main.py b/cromulant/main.py
index 70250d9..5c5b5a4 100644
--- a/cromulant/main.py
+++ b/cromulant/main.py
@@ -1,5 +1,11 @@
 from __future__ import annotations
 
+import os
+import sys
+import fcntl
+import tempfile
+from pathlib import Path
+
 from .config import Config
 from .utils import Utils
 from .ants import Ants
@@ -11,6 +17,24 @@ from .filter import Filter
 
 def main() -> None:
     Config.prepare()
+
+    program = Config.manifest["program"]
+    title = Config.manifest["title"]
+
+    pid = f"{program}.pid"
+    pid_file = Path(tempfile.gettempdir(), pid)
+    fp = pid_file.open("w", encoding="utf-8")
+
+    try:
+        fcntl.lockf(fp, fcntl.LOCK_EX | fcntl.LOCK_NB)
+    except OSError:
+        Utils.print(f"{title} is already running.")
+        sys.exit(0)
+
+    # Create singleton
+    fp.write(str(os.getpid()))
+    fp.flush()
+
     Utils.prepare()
     Window.prepare()
     Ants.prepare()