Technical Abstract: During replicated bioassays, 52 percent of larvae sprayed with spores of a virulent Paecilomyces fumosoroseus isolate became infected but only 0.7 percent inoculated with the same dosage of an avirulent isolate developed a positive infection. Our objective was to determine whether the virulent P. fumosoroseus isolate attaches to, germinates on, or penetrates cuticle of diamondback moth larvae (plutella xylostella) faster or more abundantly than the avirulent isolate. Attachment and percentage germination of spores of the virulent isolate were significantly higher than for spores of the avirulent isolate. Larvae inoculated with the virulent isolate developed dark spots on the cuticle that varied in size, number, and location. Larvae inoculated with the avirulent isolate did not develop these cuticular spots, though sporadic small dark flecks were visble on some larvae when magnified. We hypothesize that this difference in melanotic response of the cuticle to the different isolates reflects a reduced ability of the avirulent isolate to penetrate the cuticle. We intend to compare penetration and subsequent invasion of the hemocoel by the different isolates.