diff --git a/logger.php b/logger.php
index 02a5e3b..1383602 100644
--- a/logger.php
+++ b/logger.php
@@ -234,7 +234,7 @@ class logger
     ini_set ("date.timezone", $this->timezone);
     $message = date ("Y/m/d H:i:s")." [".$this->priorities[$priority]."] ".
                $message."\n";
-    file_put_contents ($this->logfile, $message, FILE_APPEND);
+    file_put_contents ($this->logfile, $message, FILE_APPEND|LOCK_EX);
   }
 
   /** Log $message on screen with adding date. Add the HTML flags if not in CLI.