The analysis of urine for dialkyl phosphate residues provides a measure of mammalian exposure to organophosphate pesticides. A method is presented for quantitative determination of six alkyl phosphate urinary metabolites. These metabolites are as follows: 0,0-dimethyl phosphate, 0,0-diethyl phosphate, 0,0-dimethyl phosphorothionate, 0,0-dimethyl phosphorodithioate, 0,0-diethyl phosphorthionate, and 0,0-diethyl phosphorodithioate. A screening method is also given for a rapid assessment of human exposure. Pentafluorobenzyl bromide is utilized as the derivatization reagent to form pentafluorobenzyl esters. The reaction products are determined by gas chromatography on routine pesticide columns utilizing the phosphorus specific flame photometric detector. Recoveries of all six dialkyl phosphates are greater than 90% with the minimum level of detection ranging from 0.04 to 0.13 ppm.