This paper describes effects of a sublethal (1 .2 mg 1-' ) organophosphate, malathion, on the ovary of an air breathing catfish, Heteropneustes fossilis . The study focuses on microscopic changes that occur on ovigerous lamellae, oocytes at different stages of development and the nucleus of the immature oocyte . Also, change in estrogen levels in blood serum is investigated . Clumping of cytoplasm appears after 24 h of exposure to malathion . Clumping intensified after 48 h. Degeneration in the follicular cells was also observed . After 72 h exposure the number of nucleoli increased, nuclear materials shrunk, oocytes became adhered . With 96 h of exposure, nuclear materials of all the oocytes shrunk to a smaller clump . The oocytes fused together, and follicular epithelium became loose and ruptured . A few atretic oocytes were visible . Radioimmunoassay of the estrogen level in blood serum after 72 h of exposure of malathion showed a reduction in the level . This study showed that the histopathological condition of the gonad is reflected in malfunctioning of the endocrine system and hormonal disbalance .

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