A disease due to lower fungus was found in pet fish, dwarf gourami (Colisa lalia). In almost all cases, swelling of the abdomen due to swollen internal organs without accumulation of ascitic fluid was characteristic external sign of infected fish. Histopathological examination showed fungal hyphae and many granulomas present in the internal organs and musculature. Fungi were isolated from the lesions and were identified as a member of the genus Aphanomyces. The fungus had high pathogenicity to goldfish (Carassius auratus). No oogonia were produced in hemp seed cultures. This fungal infection closely resembled in pathology the mycotic granulomatosis which is known in cultured ayu in Japan.