Parasites are often considered an unavoidable cost of living in groups. We examined this hypothesis investigating the ectoparasite fauna of afree-living alpine marmot population, a highly social sciurid species. Only a single important ectoparasite was found, the mite Echinonyssusblanchardi. Heavy infestation with this mite correlated with reproductive success of marmots in two ways. Infant winter mortality increased withectoparasite load, probably because ectoparasites reduced the ability of parents and alloparents to warm infants during hibernation. Litters producedby females exposed to a high number of mites during the preceding winter were weaned later. Late weaners had a lower chance of survivinghibernation. However, we found no relationship between ectoparasite load and group size or marmot density. Instead, the number of E. blanchardiper animal was lower in areas where marmot groups were clumped, i.e., in favorable habitats. Although ectoparasites most likely decrease thefitness of alpine marmots to some extent, they should not be considered a cost of social life in this species.