Technical Abstract: Both azadirachtin and Paecilomyces fumosoroseus have been used to control the silverleaf whitefly (Homoptera: Aleyrodidae), 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 products might be more effective for whitefly control if used together, different rates of each were combined in laboratory bioassays in a factorial experimental design. Both tank mixes and separated sprays were tested. Up to 90% nymphal mortality was obtained when both compounds were used, which was an increase over that seen when only one product was used, where mortality was never greater than 70%. However, the combined effects were less than additive. Azadirachtin had moderate, inhibitory effects on growth and germination of P. fumosoroseus, which may explain this antagonism.