First-order biotic degradation rate constants of 14 pesticides were determined in estuarine water and sediment/water slurry systems. Test systems used environmentally realistic concentrations of pesticides in sterile and nonsterile samples of water and sediment taken directly from the field. Thiobencarb, sulprofos, chlorothalonil, diclofop-methyl, fenthion, oxyfluorfen, methoxychlor, phorate, and trifluralin all showed significantly (p < = 0.01) more degradation in the presence of nonsterile sediment than in the presence of sterile sediment. Most of these nine pesticides biodegraded significantly faster in flasks containing sediment than in those with water alone. Endosulfan and PCNB, however, biodegraded faster in the absence of sediment. EPN and chlorpyrifos were degraded primarily by abiotic processes. Methomyl did not significantly degrade under any test conditions. Oxyfluorfen and chlorpyrifos were also slow to degrade, with half-lives of generally over two weeks in nonsterile sediment. Diclofop-methyl and phorate were the least persistent, with half-lives of a few days or less.