The Niger Delta region of Nigeria has a high concentration of companies using organo-phosphorus compounds with workers who have been exposed to these compounds for varying lengths of time resulting in toxic manifestations in some of the workers. Studies have revealed that plasma cholinesterase and red cell cholinesterase levels are reduced following organo-phosphorus toxicity. This study looks at cholinesterase levels in plasma and red cell in the occupationally exposed groups and control with a view to determining which will be more useful in the early detection of organo-phosphorus toxicity. Fourty seven subjects with early symptoms of organo-phosphorus toxicity who have worked for more than 4 years with organo-phosphorus compounds were selected from five companies localized around the same area in Port Harcourt. Fifty subjects who do not work directly with organo-phosporus compounds from the same companies as the 47 workers above but not working directly in the organo-phosphorus units were also studied while 50 persons working in the civil service were recruited as controls. The kinetic colorimetric method was used for measuring plasma and red cell cholinesterase activity Result: The results showed significant reduction in the mean plasma cholinesterase activity in the chronically exposed subjects (4614. +/- 532) who showed symptoms of toxicity when compared with that of the control group (8095 +/- 575). There was no statistical difference in the mean red cell cholinesterase activity of the chronically exposed group (7998. +/- 948) when compared with that of the control (8115. +/- 712) P > 0.05 CONCLUSION: The effect of organo-phosphorus poisoning occurs early and is more marked in plasma cholinesterase than in the red cell cholinesterase. Plasma cholinesterase is thus more useful in early detection of organo-phosphorus toxicity than red cell cholinesterase.