An occupational study was conducted for a firm employing 22 pest control operators (PCOs) exposed to three organophosphorus insecticides. Measurements of 8-hour exposure levels were less than: 131.0 µg/m³ for Vaponite® ; 41.0 µg/m³ for Diazinon® ; and 27.6 µg/m³ for Dursban ®. Twenty-four-hour urines analyzed for alkyl phosphates showed the presence of metabolites of these three pesticides. The effect of this exposure is reflected by a statistically significant inhibition of plasma acetylcholinesterase (AChE) among the PCOs as compared to an age and sex-matched control group. There were no significant differences in the mean RBC AChE values of either group. Although physical examinations detected no apparent toxic effects in the study group, biological sampling results indicated a need for personal protective equipment during the handling and application of these pesticides.