Infracommunity data from 60 perch collected from Garner Lake, Alberta, in 1992 were examined to determine whether ontogenetic shifts in host diet or habitat could produce a nested subset pattern of infracommunity structure. The host by parasite matrix showed significant nesting. Host idiosyncratic temperatures, which are indicative of differing “biogeographic histories,” were determined primarily by the presence of Ergasilus caeruleus in depauperate communities, or its absence in richer communities, and covaried positively with host age and the associated variables of host length, mass, and infracommunity richness. Idiosyncratic host temperatures did not differ significantly between male and female perch when the effect of age was controlled for by analysis of covariance. Although an ontogenetic diet shift can be ruled out as producing the observed nested pattern, it is possible that the observed nested subset pattern is the result of an ontogenetic habitat shift.