Conidial suspensions of Beauveria bassiana (Balsamo) Vuillemin and Paecilomyces fumosoroseus (Wize) Brown & Smith were tested for pathogenicity to third-instar nymphs of Trialeurodes vaporariorum (Westwood) reared on cucumber and tomato plants. Nymphs were highly susceptible to infection by both fungi after a one-time application of conidia onto cucumber plants. In contrast, insects reared on tomato plants were significantly less susceptible to infection. We hypothesized that the glycoalkaloid tomatine might have been involved in antimicrobiosis on tomato leaves. Tomatine mixed with Noble agar at five concentrations was tested for its effects on germination of conidia of both fungi. Germination of conidia of B. bassiana was only slightly affected at the two highest concentrations of tomatine. In contrast, germination of conidia of P. fumosoroseus was completely inhibited at 500 and 1,000 ppm of tomatine. The in vitro tolerance of tomatine by B. bassiana contradicted our in vivo data. Sequestered tomatine by T. vaporariorum nymphs would explain, at least partially, the insect’s defense against the pathogens. That little in vitro inhibition of B. bassiana was found supported the hypothesis that B. bassiana was inhibited only in vivo, after the penetration process. Inhibition of P. fumosoroseus might have occurred on the insect’s cuticle before penetration, as evidenced by the complete inhibition of spore germination in vitro in the presence of tomatine at 500 and 1,000 ppm. An explanation for the differential in vitro sensitivity of B. bassiana and P. fumosoroseus to tomatine is being sought.