Chlamydia psittaci (strain M56, the agent of epizootic chlamydiosis of muskrats and hares) was highly lethal for the snowshoe hare (Lepus americanus) following intravenous inoculation, whereas the agent was much less virulent for cottontail (Sylvilagus floridanus) and albino domestic rabbits (Oryctolagus cuniculus). Tissue titres of strain M56 were generally higher after 96 hr in the snowshoe hare than in the tissues of the other lagomorphs. Spleen, liver and bone marrow were apparently the chief sites of primary multiplication of strain M56 in the hare. Virulence appeared to be very host specific in that only strain M56 among the six chlamydiae tested was highly lethal for the snowshoe hare.