The biomarkers acetylcholinesterase (AChE) and glutathione S-transferase (GST) were measured in fourth-instar Chironomus riparius Meigen larvae exposed to the organophosphate insecticide pirimiphos methyl (0, 5, 10, and 50ng/g) for 48 or 96h, and at high or low food ration. Larvae exposed to 50ng/g pirimiphos methyl died within 48h. The weight of larvae exposed to 10ng/g pirimiphos methyl was significantly lower than those exposed to 0 and 5ng/g. AChE activity was significantly reduced in larvae exposed to 10ng/g, but GST activity remained unaffected. Lower food ration reduced larval weights across all treatments but did not affect biomarker measurements. Insecticide exposure was associated with a longer time to adult emergence and oviposition, fewer egg masses, a greater proportion of deformed egg masses, and fewer eggs.