Ground application of ULV aerosols of phenothrin (3-phenoxypheno) methyl Cis,trans (+)-2,2-dimethyl-3-(2-methyl-1-propenyl) cyclopropanecarboxylate alone and combined in a 1:1 ratio with piperonyl butoxide, a-[2(2butoxyethoxy)-ethoxy]4,5(methylenedioxy)-2-propyltoluene were tested against caged Aedes taeniorhynchus (Wiedemann), Anopheles quadrimaculatus Say, Musca domestica L., and Stomoxys calcitrans (L.). Based on calculated effective dosages for 90% control, there were definite differences in species susceptibility to phenothrin. S. calcitrans was the most susceptible followed in decreasing susceptibility by An. quadrimaculatus, M. domestica, and Ae. taeniorhynchus. Piperonyl butoxide had a varied effect in reducing the amount of phenothrin needed for 90% control. It was most effective against caged Aedes taeniorhynchus (Wiedemann), Anopheles quadrimaculatus Say, Musca domestica L., and Stomoxys calcitrans (L.). Based on calculated effective dosages for 990% control, there were definite differences in species susceptibility to phenothrin. S. calcitrans was the most susceptible followed in decreasing susceptibility by An. quadrimaculatus, M. domestica, and Ae. taeniorhynchus. Piperonyl butoxide had a varied effect in reducing the amount of phenothrin needed for 90% control. It was most effective against M. domestica, followed by An. quadrimaculatus and S. calcitrans. However, combination with piperonyl butoxide increased the amount of phenothrin needed for 90% control of Ae. taeniorhynchus.