The paper presents the results of investigations into the effect of 4-chloro-2-methylphenoxyacetic acid (MCPA) derivative on organic pollutants biodegradation and nitrification. Conventional activated sludge process was used. MCPA derivative concentration was 40 mg(a.i.)?L-1. Control without pesticide addition was run concurrently. Sedimentation properties of sewage sludge were bed during process however COD was degraded to less than 22 mg?L-1- and 65 mg?L-1 for control tank and tank with MCPA derivative addition, respectively. Nitrification rates were 0,0372 ? 0,0139 [mgN/mgd.m.?d] and 0,0358 ? 0,0102 [mg N/mgd.m?d] for the control and investigated system, respectively. Pesticide addition inhibited nitrification and biodegradation process rate only at about 5-10%.