1. Baculoviruses are often highly pathogenic to their insect hosts, but theoretical studies have shown that sublethal infections may also play an important role in host population dynamics. Previous experiments have suggested that deleterious effects are caused by sublethal infections in a number of host-pathogen interactions, but these studies have been criticized on the basis of the methodology used. 2. Using an improved bioassay technique, the dose- and age-dependent response of the Indian meal moth, Plodia interpunctella, to sublethal levels of its granulosis virus was investigated. For the first time, significant sublethal effects have been unequivocally demonstrated in the development rate and reproductive capacity of the host