Striped bass (Morone saxatilis) juveniles raised in an intensive culture system had chronic mortality resulting from infections with Mycobacterium marinum. Approximately one-half of a population of 900 yearlings succumbed to the disease and 80% of those remaining were infected. The bacteria were isolated on Petrignani's medium after 7 days at 25 C and subcultures grew at temperatures from 20 to 37 C. The disease was characterized by systemic nodular lesions in all major organs. Older tubercles contained numerous acid-fast bacilli. Chemotherapy by feeding rifampin (6 mg/100 g of food for 60 days) was not an effective treatment. Subclinical mycobacteriosis in adult striped bass may be the source for vertical transmission to their progeny.