Environmental chemical fumaronitrile that disrupts endocrine function has been linked to adverse effects, particularly on the reproductive system in Oreochromis mossambicus. Although there are several mechanisms through which fumaronitrile might alter the endocrine system, chemicals that mimic steroid hormones through an interaction with the estrogen receptor continues to receive considerable attention. The sex steroid hormones in Oreochromis mossambicus exposed to fumaronitrile were analysed by enzyme linked immmunosorbent assay (ELISA) to find the hormonal changes. The experiment was carried out using commercially available ELISA kit procured from Dimetra, Italy. The fumaronitrile-exposed Oreochromis mossambicus showed a significant (P < 0.05) reduction in testosterone (T) concentration when compared to control fish whereas the estradiol (E2) level in female fishes was not reduced by the fumaronitrile exposure both in plasma and ovary. Aromatase activity was significantly increased (P < 0.05) on the 100^th day in testis (11%) and ovary (16%) by the fumaronitrile in the gonads of treated fishes as compared to the control fish. Hepatic vitellogenin (VTG) production was significantly (P < 0.05) increased in fumaronitrile-treated fish. Gonadal VTG was significantly higher in male and female fish exposed to fumaronitrile. This study demonstrates that fumaronitrile has estrogenic effects in Oreochromis mossambicus and can induce VTG protein concentrations, indicating disruption of endocrine homeostasis.