WHEN Ed Caldwell, an engineer, saw a Big Mouth Billy Bass at his father's house recently, he got an idea. He borrowed the fake fish and took it to his office at Cymbolic Sciences in Bellingham, Wash. ''I put it on my co-workers' desks early in the morning,'' he said. ''When they came in it would start singing and wiggling around -- it scared them to death, but they thought it was hysterical.''

Big Mouth Billy Bass is an eerily realistic -- and multitalented -- 13-inch rubber and plastic largemouth bass mounted on a trophy plaque. Push a button or trigger a motion sensor and the fish moves its head, tail and mouth while singing ''Don't Worry, Be Happy'' or ''Take Me to the River.''

The gag gift first appeared in April at sporting goods stores and was an instant hit. At the Bass Pro Shops Outdoor World in Grapevine, Tex., 500 units sold in under an hour. ''I had friends calling me to ask if I could sneak them higher up on the waiting list,'' said Rachel Rickman, the store's events and promotions manager.

It didn't take long for the fish to reel in more than anglers. Billy Bass is now singing at passing shoppers in Wal-Mart, Kmart, Toys ''R'' Us and Spencer Gifts, among other stores. Queen Elizabeth keeps hers on the grand piano at Balmoral Castle. Billy has appeared on at least two national talk shows.