Abnormal characteristics studied were taillessness, triple-tailed, crookedback, albinism, and piebaldness in channel catfish (Ictalurus punctatus) and missing dorsal rm in blue tilapia (Tilapia aurea). Tailless catfish were divided into 2 groups: (1) totally tailless with the caudal rm completely missing and (2) partially tailless with the caudal rm reduced in size and modified in shape. Results indicated that the absence of the caudal rm in channel catfish severely reduced body weight and total length and should be selected against. Crooked back abnormality reduced body weight by 28 and 39% at 4 and 12 weeks of age, respectively. Triple-tailed catfish, however, were comparable in body weight to normal catfish at 4 and 12 weeks of age. Tailless, partially tailless, and normal catfish did not rank differently under different culture conditions indicating that multiple-environmental testing will not be required to determine relative growth of the 3 phenotypes. Albinism in channel catfish was determined to be inherited as a monogenic autosomal recessive trait. Both pied channnel catfish studied were female but differed in their color pattern and growth potential. Dorsal fin abnormality in blue tilapia was not sex influenced and had no detrimental effect on growth.