Vertebral deformity in hatchery-reared black sea bream (Acanthoparus schlegelii) fingerlings occurred. Deformed fish had a good appetite but no clinical signs were found except the vertebral abnormality and darkness around the dorsal skin of the deformed vertebra. As more than 90% of the hatchery-reared fish exhibited vertebral abnormality, the fingerlings could not be used for commercial seeds any more. No morphological change in the swim bladder was observed. Histopathological changes on the deformed vertebra, gill, liver, and spleen were observed to clarify the cause of the deformity. The vertebra were irregularly deformed upward with the shape of "V" in an alphabetic character causing the spinal cord and dorsal aorta suppressed. The diameters of the muscle fibers around the deformed vertebra, especially, in the dorsal part, were much smaller and more irregular than those in normal fish, and the gabs between the fiber bundles were enlarged. No evidence of inflammatory responses in the lateral musculature were found. On the basis of normal inflation of the swim bladder, cultural environments for growth of the fingerlings, and histopathological alterations in the muscle, vertebra and gills, it is suggested that high speed of water current in the culturing aquaria between 23 to 30 days after hatching was responsible for the development of vertebral abnormality.