In the hybrid zone of the two mouse subspecies Mus musculus musculus and Mus musculus domesticus, mice with hybrid genotypes harbour, on the average, more helminth parasites (cestodes and nematodes) than mice of the two parental taxa. In order to determine the roles played by genetic parameters in this phenomenon, mice with recombined and parental genotypes were experimentally infected with the intestinal pinworm Aspiculuris tetraptera, a natural parasite of the house mouse. The results showed that the high susceptibility of the hybrid zone mice is genetically determined. In addition, this study shows the occurrence of variability among resistant parental populations.