Abstract: In Nigeria, the analytical grade formalin is indiscriminately used in the treatment of parasitic infestation in all fish species whereas paracide F (formalin plus methanol) has been approved for use in the USA, only on the eggs of salmonids and esocids. This study was carried out to determine the effect of therapeutic dose of formalin on spawning success and the histologcal changes induced in the organs of Clarias gariepinus. Two broodstock each of both sexes were used with each sex represented in both control and formalin treated fish. Treatment involved a bath method of exposure of treated fish to 2 mL of 37% formalin in 20 L of water on consecutive days for a maximum of three treatments with aeration at all times. Control fish were exposed to culture water. Weight, length and water quality parameters were determined and spawining was artificially induced with ovupin®. Fish were stripped and eggs collected and examined microscopically. Following fertilization; hatching and quality of hatchlings was determined. Histological sections of skin, liver, spleen and testes of control and formalin treated broodstock were made and examined microscopically. The eggs from formalin-treated broodstock had clumped edges and irregular shape compared to the normal well differentiated edges with round ovoid shape seen in eggs from the control. Eggs of formalin treated fish did not hatch 24 h after fertilization, while control did. Histological lesions observed in organ samples from formalin treated fish include generalized massive vacoulations of the skin, multifocal necrosis of hepatocytes, massive lymphoid depleted spleen and seminiferous tubules. Organs of the control fish revealed normal histological architecture. Utmost caution and minimal dosage is hereby recommended if formalin is required as a therapeutic agent in aquaculture.