Parasites can exert a wide range of negative effects on their hosts. Consequently, hosts that can resist infection should have a selective advantage over nonresistant conspecifics. Yet, host populations remain susceptible to some parasites. Could genetic heterogeneity in the host's ability to resist parasites reflect costs of mounting an immune response? Previous work on Drosophila melanogaster establishes that maintaining the ability to mount an immune response decreases larval competitive ability. Moreover, mounting an immune response decreases fitness. I report on the impact of mounting an immune response on fitness of D. melanogaster survived parasitism by Asobara tabida. I used isofemale lines to determine whether genotype influences the costs of immune response. I examined fitness consequences both to larvae and adults. Survivors of parasitism show no measurable decrease in larval fitness (development time) but have decreased adult fitness (population growth rates), probably because of their smaller size. none