Abstract A coulometric microdevice based on plug-based microfluidics was developed for the detection of organophosphate pesticides (OPs). Detection was based on the inhibition of an enzyme, acetylcholinesterase (AChE), by the OPs. Hydrogen peroxide (H2O2), produced in a series of enzymatic reactions involving AChE and choline oxidase (ChOx), was oxidized on a microelectrode array, and changes in AChE activity upon addition of an OP were measured by coulometry. A linear relationship was confirmed between the charge generated and the logarithm of the OP concentration. The lower limit of detection (LOD) was 33nM for malathion and 90nM for acephate, MEP, and diazinon. An increase in the ratio of organic solvents was also found to inhibit AChE activity. However, the concentrations sufficient to produce this effect were several orders of magnitude higher than those of the OPs, indicating that this influence can be ignored in the analysis of OPs.