A coupling system for connecting or attaching a working tool to the free end of a working arm. For example, a bucket, hammer, etc. can be easily attached to the end of an articulated arm on an excavator. The coupling system includes a fixed jaw and a movable jaw which is pivotable between open and closed positions. The movable jaw includes a tab which extends through an opening in the coupler housing. A single pin is used to lock the movable jaw in its closed position.