The blood of 76 cacao farmers from Southwestern Nigeria was monitored for erythrocyte cholinesterase enzyme activity (AChE) and hemoglobin values before and after insecticide application. Eight farmers had 30% to 50% baseline AChE activity, which suggests chronic organophosphate insecticide poisoning. AChE activity inhibition suggestive of occupational exposure (20% to 30%) was manifested by 28% of the farmers, whereas 30% to 50% inhibition suggestive of hazard was manifested by 11%. Significantly depressed post-insecticide application hemoglobin values were similarly recorded among the farmers. AChE activity inhibition, depression in hemoglobin values, and the years of involvement of the farmers in insecticide application on cacao, were positively correlated. Occupational exposure hazard due to organophosphate insecticides is therefore real among cacao farmers in Southwestern Nigeria. Regular biomonitoring of their blood for AChE activity and hemoglobin level is necessary.