Mycoplasma arginini was isolated from the pharynges and tonsils of 4 of 6 swine necropsied during an epizootic of respiratory tract diseases and lameness. The organism was also isolated from 2 of 16 asymptomatic swine examined in the same herd approximately 10 months after the epizootic. In addition, M arginini, Acholeplasma laidlawii, and other acholeplasmas were isolated from recycled effluent of an anaerobic lagoon used for disposal of waste from the swine herd. This observation represents the 1st report of a Mycoplasma species isolated from a lagoon or sewage system. The pathogenicity of a strain of M arginini isolated during the epizootic was examined by inoculating young swine intranasally, intratracheally, intravenously, and intraperitoneally. The agent colonized in the pharynges of 2 pigs inoculated intranasally, but did not produce overt signs of clinical disease. Although neutralizing antibodies to M arginini were not detected, 1 animal that had been infected naturally and 2 that had been inoculated intravenously did develop complement-fixing antibodies. The findings indicate that M arginini can colonize in the nasal and pharyngeal mucosae of swine, but that it is not highly pathogenic for these animals.