The impact of ULV resmethrin on urban Culex mosquitoes was evaluated in 4 field trials by monitoring daily oviposition rate. A well-defined oscillation of effect, with a period corresponding to the duration of the gonotrophic cycle, was observed. We postulate that this oscillation arises from changes in susceptibility following blood feeding and/or behavioral factors. The data indicate that a single treatment with ULV may be inadequate for the effective control of vector mosquitoes.