Congresox talabonoides, the yellow conger pike eel, caught near Hong Kong, shows a high incidence of abnormal vertebrae and dorsal fin rays. Of 423 C. talabonoides examined during 1967-69, 244 (57.7 per cent) had a total of 999 macroscopically enlarged vertebrae, but 100 of each of the related species Muraenesox bagio and M. cinereus were unaffected. The gross and histological changes were investigated in 20 freshly caught specimens. Small eels (less than 80 cm) were not affected. Bigger specimens showed symmetrically enlarged, densely calcified vertebrae and fin rays, with more bones affected as the fish grew larger. X-rays showed that the calcification was uniform; various degrees of change were seen in X-rays of the fin rays, but the vertebrae appeared either highly radio-dense or normal. Histological examination showed deposition of dense cancellous lamellar bone, which appeared organized rather than random. In the case of the fin rays, the new bone surrounded each ray, but in the vertebrae the normal structure was replaced, then expanded. There is no clearly comparable pathology in mammals. Similar lesions in other species of fish have been attributed to toxins, but the incidence in C. talabonoides seems exceptional.