||1. Eleven species of wild rodents and one species of cottontail were found to be extremely susceptible to subcutaneous infection with the virulent Schu A strain of P. tularensis. In addition, grasshopper mice, wood rate, and/or deer mice were very susceptible to subcutaneous infection with any of the 12 strains, of P. tularensis isolated from rabbits, rodents or ticks, and one strain isolated from a horse. All three species were completely resistant to inoculation with the avirulent 38 strain. Deer mice and wood rats were completely resistant to the Russian NIIEG (gray variant) and partially resistant to NIIEG (blue variant). 2. Jack rabbits of the subspecies L. c. deserticola were highly susceptible to subcutaneous infection with the Schu A strain, whereas the subspecies L. c. texianus showed some resistance. Complement fixing, but not agglutinating, antibodies were demonstrated in the sera of L. c. texianus surviving experimental infection. 3. Of five species of rodents exposed to subcutaneous infection with the moderately virulent Jap4 strain of P tularensis, only the grasshopper mouse was very susceptible. Wood rats were completely resistant, while deer mice and kangaroo rats were lethally susceptible only to high does. 4. Tularemia infection in young coyote pups caused death when massive doses were inoculated. However, subcutaneous and oral infection usually resulted only to production of antibody. The disease followed a mild, subacute course and was completely resolved within 2-3 weeks. No carrier state or chronic infection resulted. 5. The presence of citrulline ureidase enzyme system in P. tularensis strains of high virulence and its absence of avirulent strains and strains of low virulence has been confirmed. 6. The only wild strains of P. tularensis tested which lacked a citrulline ureidase system were the two isolated from rodents or rodent ticks. All strains isolated from rabbits, rabbit ticks, a human, and a horse, that were tested, possessed this system. 5. The citrulline ureidase system has been shown to be no directly related to virulence. 8. The existence of two North American strains of P. tularensis has been postulated on the bases of the presence or absence of a citrulline ureidase enzyme system. It has been further postulated that the strains lacking this enzyme system may have evolved in rodents and the strains possessing the ability to metabolize citrulline may have evolved in rabbits.