We report cytologic observations and isolation of Mycoplasma felis in September 1992 from the lower respiratory tract of a 3-week-old captive serval (Felis serval) cub with pneumonia, in Florida (USA). Septic, neutrophilic inflammation with a large, monomorphic population of unique, pleomorphic, intracellular and extracellular rods was diagnosed from a transtracheal wash. Mycoplasma felis was the only bacterium isolated in significant numbers from the transtracheal wash.