Entomopatogenic fungi are a promising alternative for the control of agricultural pests. We evaluated the pathogenicity of three isolates of Paecilomyces fumosoroseus (Wize) Brown and Smith (Pf₁, Pf₂, and Pf₃) and six isolates of Lecanicillium (= Verticillium) lecanii (Zimmermann) Zare and Gams (VI₁, VI₂, VI₃, VI₄, VI₅, and VI₁₀) on eggs, second, third, and fourth nymphal instars of the greenhouse whitefly, Trialeurodes vaporariorum (Westwood). Three of the nine isolates (Pf₁, VI₃, and VI₅) produced 11–26% infections of eggs, while the remaining isolates had no significant effect (Pf₃ and VI₁₀) or were unable (Pf₂, VI₁, VI₂, and VI₄) to infect this developmental stage. Pf₁, Pf₂, and Pf₃ caused homogeneous pathogenicity in all nymphal instars tested, while L. lecanii isolates showed high variability. Pf₁, Pf₂, and Pf₃ were usually the most pathogenic, but their activity correlated inversely with increasing nymphal instars. The pathogenicity (number of living nymphs recovered from treated plots) of Pf₂ on T. vaporariorum nymphs was evaluated on zucchini squash plants in two experimental trials. Survival of whitefly nymphs decreased by 19 and 28% after the first and second fungal applications, respectively, in one trial, and by 62 and 71% in the other trial. Based on these results, we concluded that the use of fungal preparations based on the Pf₂ isolate could be feasible for controlling T. vaporariorum.