The fate of two insecticides (lindane and diazinon) and three herbicides (2,4-D, 2,4,5-T, and atrazine) in municipal wastewater was investigated in pilot plant experiments. The pesticides were dosed into the wastewater to provide feed concentrations in the 2 to 10 ug/L range. The pilot plant was operated at both 4- and 10-day solids retention times (SRTs) in the first of two testing campaigns and at a 4-day SRT in the second campaign. Only lindane was found to be biodegradable to any extent. Loss of pesticides from the system due to sorption to mixed liquor solids was small. Estimated partition coefficients ranged from 0.01 to 2 L/g volatile suspended solids (VSS), whereas biodegradation rates ranged from 0.1 for lindane to 4 E-6 L/g VSS – h for 2,4,5-T. Because the pesticides examined are mostly refractory to activated sludge treatment under the conditions examined, it is desirable to limit their entry to wastewater collection and treatment systems to minimize their impact on receiving waters.