In hybridogenetic systems, hybrid individuals are fully heterozygous because one of the parental genomes is discarded from the germinal line before meiosis. Such systems offer the opportunity to investigate the influence of heterozygosity on susceptibility to parasites. We studied the intensity of lung parasites (the roundworm Rhabdias bufomis and the fluke Haplometra cylindracea) in 3 populations of water frogs of the Rana lessonae-esculenta complex in eastern France. In these mixed populations, hybrid frogs (R. esculenta) outnumbered parental ones (R. lessonae). Despite variation in parasite intensity and demographic variability among populations, the relationship between host age and intensity of parasitism suggests a higher susceptibility in parentals than in hybrids. Mortality is probably enhanced by lung parasites in parental frogs. On the other hand, while parental frogs harboured higher numbers of H. cylindracea than hybrid frogs, the latter had higher numbers of R. bufonis. Despite such discrepancies, these results support the hybrid resistance hypothesis, although other factors, such as differences in body size, age-related immunity, differential exposure risks and hemiclonal selection, could also contribute to the observed patterns of infection.