The effect of the entomopathogenic fungus Lecanicillium longisporum on the parasitoid Diaeretiella rapae was evaluated under laboratory conditions. M. persicae nymphs were first exposed to 1-day-old mated parasitoid females for 24 h and then sprayed at different intervals with a solution of L. longisporum. The aphids were primarily sprayed with a solution of the entomopathogen and then exposed to parasitism at different time intervals. The parameters evaluated were parasitism rate, emergence rate, the period between oviposition until mummification, the period between oviposition until emergence, sex ratio (F1 generation), and longevity of F1 generation females. Parasitism was reduced from 66.47 to 17.75% from previous spraying of the entomopathogen on M. persicae. Parasitoid emergence was affected in treatments consisting of spraying L. longisporum immediately (0 h) after or before parasitism. Decreases in the longevity of adult females from the D. rapae F1 generation were related to the presence of the entomopathogenic fungus. The results indicate that there are two biological control agents that can be used in IPM systems against M. persicae, but this relationship requires some caution in order to avoid antagonistic interactions in the field.