Beauveria bassiana (Balsamo) Vuillemin is an important entomopathogenic fungus of Bemisia tabaci (Gennadius) biotype B. The pathogenesis of an isolate (C#1) of B. bassiana in B. tabaci was observed on the body using scanning electron microscopy, whereas the development of the fungus inside the whitefly body was studied using light microscopy with histological sections of infected insect bodies. The infection process was closely associated with the specific regions of the whitefly nymph surface. Conidia accumulate in high densities on the intersegmental membranes. In the vasiform orifice area, conidia seldom adhere to the cuticle except in areas adjacent to the dorsum, but seldom (6.3% ± 1.2) germinate. Germinating conidia and some germ tubes with mucoid materials were seen in the lateral areas 10 h (85.0% ± 4.5) after conidia application. Appresoria began to form on the cuticle in 12 h, and more appressoria were formed on the dorsum and the intersegmental membrane areas of the dorsal surface. Germinated conidia could be observed in the same area 24 h later. Penetration of the cuticle occurred 36 h (79.2% ±6.9) after conidia application. After germination, the germ tubes oriented to the cuticle and penetrated quickly. Fungal blastospores were observed under the cuticle 36 and 48 h after inoculation and eventually filled the body of the nymph. The nymphs died by the time a red pigment appeared in their bodies, ≈96 h after inoculation. By 120 h after treatment, large quantities of conidia were found on the surfaces of the whitefly nymphs.