When posttreatment response to ultra low volume (ULV) application of insecticide has been followed for periods approaching a week, a damped oscillation in oviposition rates is observed, probably because females who have been recently blood-fed are more resistant to insecticides than their unfed siblings. We describe a simple model (ULVSIM) that incorporates physiologic changes in insecticide susceptibility and accounts for much of our field data. The model follows 30 cohorts over 30 days following insecticidal treatment. Multiple treatments or short-term residual activity can be evaluated. The model predicts that oviposition will follow a pattern of damped oscillations after an adulticidal treatment. The model gave a good fit to oviposition data obtained in 2 field trials of resmethrin for 7 to 9 days after treatment. It can be used to evaluate the effect of single and multiple treatments on the total female Culex population and on numbers of infected females surviving for different periods following an infective blood meal.