Technical Abstract: Conidial suspensions of Beauveria bassiana (Balsamo) Vuillemin and Paecilomyces fumosoroseus (Wize) Brown and Smith were tested in the laboratory for pathogenicity to 3rd-instar nymphs of Trialeurodes vaporariorum (Westwood) reared on cucumber and tomato plants. The nymphs were highly susceptible to infection by both fungi following a one-time application of conidia onto cucumber plants. In contrast, the insect reared on tomato plants was significantly less susceptible to infection by the fungi. We hypothesized that the glycoalkaloid, tomatine, produced by many cultivars of tomato, 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 and the 24-h IC90 (concentration that inhibited 90% of germination) value for tomatine was 3Y 10*5 ppm. In contrast, germination of conidia of P. fumosoroseus was completely inhibited at 500 and 1000 ppm of tomatine; the 24-h IC90 value was 480 ppm. Our in vivo findings on tomato pointed at a potential host plant-mediated antibiosis. The in vitro tolerance of B. bassiana to tomatine contradicted our in vivo data. The presence in nymphs of tomatine sequestered by T. vaporariorum would explain, at least partially, the defense of the insect against the pathogens. That little in vitro inhibition of B. bassiana reinforced the hypothesis that B. bassiana was inhibited in vivo, after the penetration process.