A cat was presented to a veterinary clinic with a severe pleural effusion from which an organism later identified as anActinomycesspp., probablyA. odontolyticus, was isolated along withBacteroides mehnogenicus. It was successfully treated with surgical drainage and a combination of amoxycillin and sulphadiazine/trimethoprim. The fluid was cytologically examined to obtain a provisional diagnosis and to allow treatment to be initiated while the organisms were cultured and identified. The cat is now clinically normal except for slight abdominal respiration, presumably resulting from adhesions following the severe infection.