The effect of separate and combined activity of the entomopathogenic fungus, Paecilomyces fumosoroseus Trinidadian strain T11 and parasitoid, Encarsia formosa, was assessed on populations of Trialeurodes vaporariorum (greenhouse whitefly) infesting Phaseolus vulgaris (French bean) and Pelargonium x domesticum (Regal geranium) plants to determine which treatment or combination of treatments was most efficacious. Under identical exposure conditions, bean plants were much more heavily infested by the whitefly than were the geranium plants. When infested plants were exposed to the parasitoid for 24 hours, and sprayed four days later with P. fumosoroseus blastospores (218-237/mm²), percent whitefly mortality was 97 and 63% for beans and geraniums, respectively. Treatment of infested bean plants with either E. formosa or P. fumosoroseus resulted in 80 and 88% whitefly mortality, respectively, while similar treatment of infested geranium plants resulted in 2 and 59% whitefly mortality. The effect of the parasitoid and entomopathogen on whitefly mortality was found to be the result of independent actions of each agent; synergism was not observed. Our results support the use of E. formosa and P. fumosoroseus in combination for the treatment of whitefly infested plants since higher mortality was observed after application of the entomopathogen and parasitoid, than when either biocontrol agent was applied alone. Thus, P. fumosoroseus is compatible with and can be used successfully in combination with E. formosa.