We examined the relationship between mammalian hair diameter and body mass at several taxonomic levels (interordinal, intrafamilial, intrageneric, and intraspecific) and showed a significant, positive allometric relationship between hair diameter and body size at all taxonomic levels examined. The allometric coefficient (α) ranged from 0.13 to 0.33. Within pocket gophers (Geomyidae), a significant positive relationship exists between hair diameter and rostral groove dimensions of their chewing lice, Geomydoecus, which use the rostral groove to grasp hairs of their host. Coupled with previous evidence of a strong allometric relationship between rostral groove width and louse body size, our findings suggest that hair diameter of the host is an important determinant of body size in chewing lice that parasitize pocket gophers.