Both azadirachtin and Paecilomyces fumosoroseus (Wize) Brown & Smith have been used to control the whitefly Bemisia argentifolii Bellows & Perring, but with only moderate effectiveness. Azadirachtin is a botanical insecticide derived from the neem tree, and P. fumosoroseus is an entomopathogenic fungus. To test whether these two agents might be more effective for whitefly control if used together, different rates of each were combined in laboratory bioassays in factorial treatment. Both tank mixes and separate sprays were tested. Up to 90% nymphal mortality was obtained when both the fungus and azadirachtin were combined, a significant increase over the 70%, or less, mortality obtained when only one agent was used; however, the combined effects were less than additive. Azadirachtin had moderately inhibitory effects on growth and germination of P. fumosoroseus, which may explain this antagonism.