An enzyme‐based assay was developed to monitor the levels of the organophosphate acaricide coumaphos and potasan, a metabolite formed by reductive dechlorination in cattle dipping solutions. The assay relies on the differential rates of hydrolysis of coumaphos and potasan by the enzyme parathion hydrolase (organophosphorous anhydrase [EC 3.1.8.1]). Hydrolysis of coumaphos and potasan yields the fluorescent products chlorferon and 4‐methylumbelliferone, respectively. By altering the reaction conditions and measuring the formation of fluorescent compounds in two parallel reactions (one reaction for potasan hydrolysis, one reaction for hydrolysis of potasan and coumaphos), it was possible to measure both coumaphos and potasan in samples from cattle dipping vats and from a bioreactor used for digestion of coumaphos‐containing wastes. This sensitive and reliable assay would be suited for use in remote field sites where more conventional chemical analysis is difficult to obtain.