The distribution of parasites within host natural populations has often been found to be host age-dependent. Host mortality induced by parasites is the commonest hypothesis proposed for explaining this pattern. Despite its potential importance in ecology, the parasitism intensity in relation with the host age has rarely been studied in the field. The 2 manipulative acanthocephalans, Polymorphus minutus and Pomphorhynchus laevis, use the amphipod Gammarus pulex as an intermediate host, and their infection intensity and incidence among G. pulex populations were examined by analyzing 2 large samples of hosts collected in eastern France. Both parasites had low prevalence in the host populations, but their mean abundances were highly related with gammarid age. For the 2 acanthocephalans, results reported a disappearance or an absence of heavily infected hosts in the older host age classes. These results suggested that parasites that alter intermediate host behavior for enhancing their transmission success to the definitive host reduce the survival of their intermediate host. In conclusion, manipulative parasites might act as a mechanism regulating the density of gammarid populations.