Seasonal abundance of immature Coquillettidia perturbans was studied at 3 sites in south-central Michigan in 1987 and 1988. A modified boat bilge pump and funnel separators proved to be good sampling and sorting devices. Pupae were found in late May to early July, first and second instars were most abundant in July and August, while third and fourth instars were present year-round. Overall, larvae were most numerous in August and September, indicating that Cq. perturbans has a single generation per year in Michigan, with midsummer to fall as the main period of larval development, and with third and fourth larval instars as overwintering stages. Parasitism of Cq. perturbans larvae by a previously undiscovered mermithid nematode was documented at 2 sites in late summer.