We conducted experimental infection tests of Mycobacterium marinum NJB 0419 isolated from yellowtail Seriola quinqueradiata to amberjack S. dumerili and red sea bream Pagrus major to determine the pathogenicity of the bacterium. As a result, NJB 0419 was pathogenic to both of them. In light of concerns about zoonoses, BALB/c mice were intravenously or subcutaneously injected with NJB 0419. The histopathological findings of examined mouse tissues were normal. Acid-fast bacteria were not found in the tissues and not re-isolated from the tissues. Our data suggest that M. marinum NJB 0419 might not be a strong pathogen of mammals, although more intensive studies are required for confirmation of the pathogenicity.