The levels of pesticides reaching the biological wastewater treatment plants have been increas-ing. Operational conditions leading to the most efficient removal of pesticides in these plants should be evaluated first before attempting tertiary-level treatment options. This study investi-gated the influence of solids retention times (SRTs) on the removal of three pesticides (carben-dazim, imidacloprid, and aclonifen) in the activated sludge process. Laboratory-scale reactors re-ceiving these pesticides (0-400 mu g/L) were operated at five different SRTs (3, 8, 10, 20, and 30 d). When the pesticides were present individually, all of the reactors were capable of remov-ing carbendazim up to 25 mu g/L, imidacloprid, and aclonifen up to 10 mu g/L, by almost 100%, re-gardless of SRT. When the pesticides were present in combination, higher pesticide removals were generally attained, although there was no clear correlation between SRT and removal effi-ciency. Aclonifen being the least soluble one was removed more efficiently, either individually or in a mixture, when the SRT was set to 3 or 30 d. The COD removal efficiency was not impaired at all until 50, 10, and 10 mu g/L of individual carbendazim, imidacloprid, and aclonifen levels, re-spectively, at all SRTs. When the pesticides were present in combination, each 50 mu g/L, there were no adverse effects on COD removal, provided that SRT is 30 d. The competitive inhibition of COD removal was evident. The obtained results demonstrated that the activated sludge process receiving pesticides in combination could be operated with better pesticide and COD removal performances if SRT is closely controlled.