The Satan Bug (1965)

Special Agent Lee Barrett (George Maharis) joins forces with Ann Williams (Anne Francis), a general's daughter, to recover a biological weapon stolen from a secret government lab. Learning that the pathogen could destroy all life on Earth, they focus their attention on a mysterious millionaire who may have orchestrated the theft. After annihilating a small town in Florida, the villain turns his attention to Los Angeles -- and the duo must race to stop the city's destruction.