Mutants of a marine luminous bacterium, in which the ability to emit light is conditional upon temperature, have been isolated. The mutants obtained fall into three classes, which are readily distinguishable by both in vivo and in vitro criteria. In one class an altered, more temperature-sensitive luciferase is produced; in a second, the luciferase is actually not produced at the higher temperature. The third is defective in the production of an aldehyde-like factor that is a known requirement in the in vitro reaction.