Compared to force-resisting elements of the mammalian feeding apparatus, data on jaw-muscle plasticity are less common. This hinders our understanding of the role of force-producing structures in craniofacial development and integration. Thus, we investigated fiber-type abundance and cross-sectional area in the masseter muscle of growing rabbits subjected to diet-induced variation in masticatory stresses. Three loading cohorts were obtained as weanlings and raised until adult on different diets. Immediately following euthanasia, left-sided masseters were dissected away, weighed, and then divided into anterior, intermediate and posterior sections for fiber-type immunohistochemistry. These data were compared to mandibular proportions and biomineralization from the same subjects. Results indicate that growing mammals fed a tougher, fracture-resistant diet develop: absolutely and relatively lower numbers of Type I jaw-muscle fibers; absolutely larger fiber cross-sectional areas; and relative increases in the amount of Type II fibers. These analyses indicate that an early postweaning dietary shift can induce significant variation in muscle fiber types. Such norms of reaction are comparable to those observed in bony elements. Functionally, the processing of fracture-resistant foods results in jaw adductors potentially characterized by faster contraction times and higher force production capabilities, which may influence the frequency and amplitude of forces experienced by oral tissues.