The Giant Behemoth (1959)

Starring Gene Evans, Andre Morell, John Turner, Leigh Madison, Jack MacGowran
Directed by Douglas Hickox & Eugene Lourie
(actor & director credits courtesy IMDB.com)

The discovery of countless dead fish and a gruesome murder near a British port is investigated by atomic scientists, who discover a giant radioactive sea monster is to blame.

This British piece of sci-fi, very similar in form to Lourie's earlier picture, The Beast From 20,000 Fathoms, which showcased the stop-motion animation of Ray Harryhausen, is probably of most interest for effects work by Harryhausen's mentor, Willis O'Brien.  However, the creature in this film doesn't actually show up in animated form until about an hour into the picture, with earlier shots of the monster apparently accomplished via a puppet that is not animated.  Nevertheless, it's a more than effective film, with stalwart leads in Evans and Morell, inventive sound effects, and plenty of satisfying monster action.