According to Fisher's Principle, an equal sex ratio is an evolutionary stable strategy. However, biased sex ratios have been reported in many metazoan parasite species, although the causes and mechanisms of the observed bias are still poorly understood. In the present study, we analysed sex ratios in long-term datasets from three populations of bank voles (Clethrionomys (=Myodes) glareolus) infected with Heligmosomum mixtum and Heligmosomoides glareoli. The overall sex ratios of both species were female-biased but in contrast to previous studies we did not find a relationship between the proportion of females and infection intensity. A higher female bias was observed in older hosts, suggesting that the sex ratio changes over time; the lifespan of nematodes in the family Heligmosomidae is known to be comparable with that of their hosts. We also compared the distributions of sexes in voles infected with two, three, four or five worms and we found significant differences from the expected values in both parasite species. In infections with four and five H. glareoli we observed more single-sex infections than expected, both female- and male-dominated, whereas in the case of H. mixtum female-dominated infections were more frequent.