From 65 bluefish caught in Long Island Sound, 433 isolates of bacteria from the intestinal contents were assigned to various groups, the most abundant being Vibrio, Pseudomonas, Enterobacteriaceae, and Achromobacter. Unidentified fermenters, yeasts, and strains of Flavobacterium, Micrococcus, and Bacillus were found also. In each group, two or three biotypes predominated. Flagellar arrangements and biochemical characteristics are presented for the major groups of Gram-negative bacteria isolated. The majority of cultures required sea water for growth, and grew preferentially at 20 C. Relationships between these data and studies of the intestinal microflora of other marine fish are discussed. Generic and physiological similarities and differences as well as taxonomic difficulties encountered are considered.