ONE of the main difficulties in the study of marine bacteria, and particularly of those types which constitute the flora of fresh and spoiling fish, is that of identification and classification. This is due to the negative results given by these bacteria in most common biochemical tests. It was considered that the use of bacteriophage might help with this difficulty, and consequently attempts were made to isolate bacteriophages active against certain marine bacteria associated with fish, and particularly against certain luminous bacteria probably identical with Photobacterium phosphoreum (Bacterium phosphoreum Bergey1). These luminous bacteria have been shown by various workers to constitute a considerable part of the flora of many species of fish2.