Interactions between the immature stages of Diadegma semiclausum, an endolarval parasitoid of Plutella xylostella, and the fungal entomopathogen Beauveria bassiana were investigated in the laboratory. Detrimental effects of B. bassiana on D. semiclausum cocoon production and adult parasitoid emergence increased with increasing pathogen concentration and some parasitoid larvae became infected by B. bassiana within hosts. The negative impact of B. bassiana on D. semiclausum cocoon production decreased as temporal separation between parasitism and pathogen exposure increased. Adult parasitoid emergence was significantly compromised by the highest rates of B. bassiana tested even when exposure of host larvae to the pathogen was delayed until one day before predicted parasitoid cocoon formation. Parasitoid pupae were infected by the pathogen in all B. bassiana treatments which did not preclude their development. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Diseases and disorders; Microbial diseases Plutella xylostella (Plutellidae); Hymenopteran parasites; Diadegma semiclausum; Infection of immature stages of parasite by fungal disease; Fungal diseases; Beauveria bassiana; Infection of immature stages of hymenopteran parasite Diadegma semiclausum (Ichneumonidae); Lepidopteran hosts; Plutella xylostella; Infection of immature stages of parasite by fungal disease; Fungal diseases; Beauveria bassiana; Infection of immature stages in lepidopteran hosts none