Them! (1954)

Starring James Whitmore, Edmund Gwenn, Joan Weldon, James Arness, Onslow Stevens
Directed by Gordon Douglas
(actor & director credits courtesy IMDB.com)

Authorities in New Mexico discover atomic fallout has enlarged a colony of ants to gigantic size, and set out on a race to destroy them before they can migrate and multiply.

In my opinion, this was the best of the giant bug movies of the 1950s, and it holds up remarkably well, due to its excellent special effects and the serious performances of the distinguished cast, particularly Whitmore and Gwenn.  Although the ants are giant mocked-up puppets which are not animated and did not have a wide range of movement, they look very fearsome as manipulated by their handlers and photographed by Sid Hickox, with eerie and deafening sound effects.  But even before the ants appear, we're treated to a mysterious and suspenseful opening of the film which ranks among the best in any science fiction picture, as a young girl in shock wanders aimlessly through a desert and policemen follow her trail to a series of violent and gruesome murders.  There's also plenty of famous names in the cast to spot, from Gunsmoke's James Arness and Davy Crockett's Fess Parker to a very young Leonard Nimoy, and familiar character actors like William Schallert, Onslow Stevens, and Richard Deacon.