Organophosphate insecticides are known to inhibit blood cholinesterase (ChE) activities in vitro and in vivo with various inhibitory kinetics outcomes. This study assessed the in vitro inhibition of human blood ChE activities by the organophosphate insecticide dichlorvos in farmworkers previously exposed to pesticides in comparison to non-exposed (control) participants, and the differences in time-dependent dichlorvos plasma ChE inhibition and its turnover kinetics were determined in the two groups. Dichlorvos at a 0.25 and 0.5 μM significantly inhibited in vitro plasma ChE activity (68%-90%), but to a lesser extent that of the erythrocytes (8%-14%) in both groups. Erythrocyte ChE activity significantly decreased (8% and 14%) in the pesticide-exposed group compared to the non-significant reductions (10% and 11%) in the control one. Dichlorvos differentially affected the rate of inhibition of plasma ChE activity between the two groups. The progress of in vitro inhibitory effect of dichlorvos with different incubation times on plasma ChE activity of pesticides exposed and non-exposed groups illustrated that the decline in plasma ChE activity of the non-exposed group was faster compared to that of exposed group. This differential effect in the pesticide exposed group was manifested by decreases in the inhibition rate constant (19.2%) and the overall rate of inhibition (44.5%) with concomitant increases in the half-life of inhibition (25%) and total inhibition time (24.4%) in comparison to the control non-exposed group. In conclusion, dichlorvos in vitro differentially inhibited plasma ChE activity compared to erythrocyte ChE activity in both groups. The calculated inhibition kinetics revealed that dichlorvos differentially also affected the progress of plasma ChE inhibition in the pesticide-exposed group. These effects might have resulted from the insensitivity of the enzyme after prior exposure to pesticides.