A diagnostic scheme employing 10 media, 4 reagents, and 5 specific antiserums has been devised for identification of gram-negative pathogens of fishes. The scheme is not intended for detailed speciation of organisms, but for generic identification of organisms associated with infected fishes. Diagnosis can be accomplished in 24 h with selective media and antiserums, and in no more than 72 h with other media and reagents.