Owen, C. R. (U.S. Public Health Service, Rocky Mountain Laboratory, Hamilton, Mont.), E. O. Buker, W. L. Jellison, D. B. Lackman, and J. F. Bell. Comparative studies of Francisella tularensis and Francisella novicida. J. Bacteriol. 87:676-683. 1964.-Comparative studies of various properties of Francisella tularensis (= Pasteurella tularensis) and F. novicida were performed. The two organisms are very similar morphologically. Growth of both was markedly enhanced by addition of cystine to media, but F. novicida is less fastidious than F. tularensis. The virulence of F. novicida for mice and cavies is lower than that of fresh isolates of F. tularensis. In complement-fixation tests, some cross-reaction occurred when rabbit antisera were used; complement-fixation tests with cavy antisera were specific. Agglutination tests with sera from both rabbits and cavies were specific. Nonliving vaccines of the two organisms (extracts, whole dead cells) conferred no cross-protection to mice; living attenuated vaccines conferred cross-protection which was more transitory than was specific protection. Passive cutaneous anaphylaxis (PCA) tests were highly specific. Absorption of antisera with homologous organisms removed all PCA reactivity, while absorption with heterologous organisms left it almost intact. Hemagglutination and hemagglutination-inhibition tests were specific. It was concluded that the two organisms are sufficiently similar to belong in the same genus but sufficiently different to be retained in separate species.