Abstract Zooplankton communities with or without Chaoborus larvae were established in outdoor experimental ponds, to which the insecticide carbaryl was applied repeatedly at 10 or 100 μg litre −1. In the ponds without Chaoborus, Cladocera dominated, but the species composition differed among the treatments. A large-sized Cladocera Daphnia galeata dominated the controls, the medium-sized cladocerans Diaphanosoma brachyurum and Moina micrura became dominant in the low-dose treatment, and the small-sized Bosmina fatalis increased in the high-dose treatment. These results indicated differential sensitivity to carbaryl among the cladocerans, and that smaller Cladocera were more tolerant of the chemical than the larger one. In the ponds with Chaoborus, rotifers dominated the zooplankton, probably because Chaoborus released rotifers from competition with cladocerans and calanoid copepods, which were eliminated by the Chaoborus predation. No effects of low-dose carbaryl treatment were detected on zooplankton communities in the Chaoborus ponds. The dominance by rotifers, organisms tolerant to carbaryl, minimized the effects. Thus, Chaoborus altered the zooplankton community responses to the chemical application by changing community structure. Repeated application of high-dose carbaryl did affect the rotifer community, decreasing the dominance of Polyarthra trigla and increasing that of Keratella valga. These rotifer species may differ in their sensitivity to carbaryl.