Variation in host susceptibility and parasite-induced mortality are preconditions for parasite-related selection on host populations. In terrestrial gastropods, variation in resistance against ectoparasite infection is poorly understood. We examined the within- and among-family variation in parasite load in full-siblings of the land snail Arianta arbustorum experimentally infected with Riccardoella limacum, a mite living in the mantle cavity of helicid land snails. We also quantified the influence of family origin and host size on parasite load and calculated its heritability (h2). Furthermore, we examined the influence of parasite load, snail size, and family origin on host winter mortality, an important life-history trait of A. arbustorum. Parasite load was heritable (h2 = 0.63). In infected snails, parasite load was affected by family origin and increased with increasing shell size. Host mortality during hibernation increased with increasing parasite load and differed among families, but was not affected by snail size. Our results show high among-family variation both in resistance against ectoparasite infection and in host winter mortality. Furthermore, we show that parasite load is linked to snail size, which suggests that the proliferation of R. limacum is limited by resources provided by A. arbustorum.