The present study was carried out to investigate the toxic effect of two of the widely used insecticides on Tilapia nilotica fish. Nuvacron; which is a fast-acting organophosphorus insecticide with both systemic and contact action, used widely against pests. Pirimor; a selective carbamate aphicide, effective against organophosphorus-resistant aphide strains. LCc of nuvacron, pirimor and their mixture using water media having pH from 6.4 to 6.8 and total alkalinity of 30 to 38 ppm as Caco, were 35, 145 and 33.33 mg/liter water respectively. The potent iation toxicity showed that the interaction between nuvacron and pirimor was synergistic in Tilapia nilotica fish. The clinical signs observ ed on the toxicated fish were recorded.