Francisella tularensis is a small, facultative, coccobacillary, Gram-negative rod withmuch the same morphology as Brucella. It is one of the few bacterial species of medical importance that does not grow on the usual enriched media. This characteristic is due to a special requirement for sulfhydryl compounds, and growth occurs best on a cysteine– glucose blood agar medium incubated aerobically. On primary isolation, 2 to 10 days of incubation is required for appearance of the tiny transparent colonies. The species is anti-genically homogeneous.