Beginning in August of 1991 biologists from the Florida Game and Fresh Water Fish Commission began receiving complaints from bass fisherman regarding the poor condition of fish and increased effort required to catch them. Largemouth bass (Micropterus salmoides) were collected from Lake Harris, a 7,000 surface acre lake north of Orlando, Florida. Fish were extremely thin and in poor condition. Necropsy was non-remarkable except for mild ecchymosis located on the anterioventral portion of the swim bladder mucosa. Bacterial cultures of swim bladder lesions consistently developed alpha-hemolytic zones, yet no bacteria were isolated on blood agar. An organism from the order Mycoplasmatales was isolated using a special media, which was a modification of a standard Mycoplasma growth medium. Efforts to identify the organism have shown that it is not an avian mycoplasma. Additional work is in progress. In October 1992, 100 captive-reared largemouth bass fingerlings were inoculated intraperitoneally with 1.3 x 104 organisms. No adverse effect on the fish was noted over a 15 month observation period.