The silverleaf whitefly is a world wide distributed pest that feeds on more than five hundred plant species and produces economically important yield losses on crops. The pathogenicity of twenty Peruvian isolates of the entomopathogenic fungus Paecilomyces fumosoroseus was screened on second stadium nymphs. Excised sweetpotato leaves with second instars attached to them were dipped in conidia suspensions and incubated for twenty days. All the tested isolates were infective and from them CIPWF24 was selected as the most pathogenic in the group. Five concentrations of CIPWF24 conidia were used to assess the Medium Lethal Concentration (LC₅₀) which was estimated on 2.24 x 10⁷ conidia/ml for the second stadium. Results indicate that the isolate CIPWF24 produced significant infection on whitefly nymphs and that this may vary according to the age of the insect.