Five phages isolated from lysogenic strains of Vibrio fetus var. venerealis and two from V. fetus var. intestinalis were tested for lytic activity on 95 V. fetus strains from various animal and human hosts. In addition, virion and plaque morphology of the seven phages were compared. Electron micrographs showed that all were the kite-tailed variety with minor variations in head and tail dimensions. Plaques of V45 and V2 were small, clear and irregular; those of V3, V8, and V19 were large, clear and regular at the edge; the plaques of V16 and V20 were intermediate in size, clear, and very irregular at the edge with satellite plaques. The number of strains lysed by one or more phages were as follows: 29 of 30 from cattle; 7 of 11 from sheep; 1 of 5 from pigs; 1 of 1 from a monkey; and 33 of 42 from human hosts. Four natural groups of phages were derived by statistical measures of percentage of similarity in lytic activity. Group III lysed more strains (46 of 95) than any of the others. Twenty-five strains were lysed by group IV, 23 strains by group I, and 19 strains by group II. Results of this study indicate that phage typing should be a practical supplement to other differential tests for V. fetus.