Large-bodied species of hosts often harbor large-bodied parasites, a pattern known as Harrison's rule. Harrison's rule has been documented for a variety of animal parasites and herbivorous insects, yet the adaptive basis of the body-size correlation is poorly understood. We used phylogenetically independent methods to test for Harrison's rule across a large assemblage of bird lice (Insecta: Phthiraptera). The analysis revealed a significant relationship between louse and host size, despite considerable variation among taxa. We explored factors underlying this variation by testing Harrison's rule within two groups of feather-specialist lice that share hosts (pigeons and doves). The two groups, wing lice (Columbicola spp.) and body lice (Physconelloidinae spp.), have similar life histories, despite spending much of their time on different feather tracts. Wing lice showed strong support for Harrison's rule, whereas body lice showed no significant correlation with host size. Wing louse size was correlated with wing feather size, which was in turn correlated with overall host size. In contrast, body louse size showed no correlation with body feather size, which also was not correlated with overall host size. The reason why body lice did not fit Harrison's rule may be related to the fact that different species of body lice use different microhabitats within body feathers. More detailed measurements of body feathers may be needed to explore the precise relationship of body louse size to relevant components of feather size. Whatever the reason, Harrison's rule does not hold in body lice, possibly because selection on body size is mediated by community-level interactions between body lice.