Abstract.Deformities consisting of an upward curvature of the spine in the caudal region with varying degrees of reduction of the caudal fin were observed in some fry of a laboratory population ofOreochromis niloticus(L.). This spinal deformity was combined with a reduction of the operculum, exposing some gill lamellae. No deformed fry survived for more than 7 days after completion of yolk sac absorption. Breeding tests demonstrate that the mutant phenotype is controlled by a single autosomal recessive gene.