Abstract: The aim of present research to study the effects of in vitro and in vivo exposure of catfish to heavy metals to determine whether the steroidogenic activity in sperm would be affected by two heavy metals contaminants, zinc and cadmium. Three groups of six male African catfish were fed from the sexually immature juvenile stage, with diet containing 1000 ppm zinc, cadmium or zinc + cadmium for 110 days and 20αHSD activity in milt of these fish were compared with six other control fish fed with normal diet. The 20αHSD enzyme activity was also measured in in vitro incubation of milt from six control fish with different concentrations (0, 0.1, 1, 3, 10, 30, 100, 1000 and 3000 ppm) of zinc, cadmium or zinc + cadmium. A very high 20α hydroxy steroid dehydrogenase enzyme activity was found in all sperm incubations of African catfish. This enzyme converts 17-hydroxy progesterone (17P) substrate to 17,20α-dihydroxy progesterone (17, 20αP) product and the rate of enzyme activity is related to substrate (17P) concentrations. Significant differences (p<0.05) in enzyme activity in converting 17P to 17,20αP were found between in vitro incubations of sperm with different concentrations of zinc, cadmium or zinc + cadmium and control group (0 ppm). Significant differences (p<0.05) in enzyme activity and 17,20αP production were found between fish fed with diet containing 1000 ppm zinc or cadmium and the group fed with diet containing 1000 ppm zinc + cadmium and control groups. The results showed that 20αHSD enzyme activities in fish sperm may be used as indicator of water contamination with heavy metals and their bioaccumulations in testis of aquatic animals.