The efficacy of malathion ULV (Fyfanon® ULV) was tested on laboratory-reared and field-collected boll weevils, Anthonomus grandis Bohman, under simulated rain and some natural rain conditions. The efficacy of malathion was not affected when applied to cotton leaves that were subjected to as much as 1" of simulated rain. However, results indicate that rain exceeding 0.125" falling within six hours after application, has reduced the levels of mortality in the adult weevils exposed to treated leave, consequently requiring reapplication.