The effects of host plant foliage on the susceptibility ofHelicoverpa zea(Boddie) toHeliothisbaculovirus (HzNPV) was examined in three separate laboratory bioassays. Larvae were administered viral suspensions, equivalent to three polyhedral inclusion bodies per insect, within 6 h of molting to the third instar for all bioassays. In the first bioassay,H. zealarvae were fed one of the following: artificial diet, cotton, grain sorghum, soybean, or tomato foliage for 24 h before and after administration of a viral suspension on the respective foliage. Larvae fed cotton were significantly less susceptible (36% mortality) than those fed either tomato or artificial diet (75% and 65% mortality, respectively). In the second bioassay, larvae were reared on artificial diet, dosed on plant foliage disks, and returned to artificial diet. No significant differences in virus-induced mortality were observed among the cotton (61%), tomato (61%), or artificial diet (63%) treatments. In a final bioassay, larvae, reared on artificial diet, were fed foliage 24 h before and after receiving virus on artificial diet disks. No differences in virus-induced mortality were observed among the cotton (69%), tomato (66%), or artificial diet (69%) treatments in the final bioassay. Overall results suggest that interaction(s) involving host plant foliar constituents and larval midgut conditions were responsible for the reduced mortality of insects fed and dosed on cotton.