House mice selectively bred for high voluntary wheel running display a mini-muscle (MM) phenotype wherein mass-specific mitochondrial enzyme activities are double that of normal, but muscle mass is reduced by half. In addition, mini-muscles are characterized by small muscle fibers in the superficial region of the plantaris and medial gastrocnemius muscles. To determine the structural alterations facilitating aerobic metabolism in these mini-muscles, cross-sections of the medial gastrocnemius muscle of normal (N; n=6) and mini-muscle (MM; n=6) mice were histo- and immuno-chemically labeled and analyzed for fiber size, capillarization, and fiber type. On the basis of the higher mitochondrial enzyme activities in muscles of MM mice, we hypothesized that they would have greater fiber capillarization in the medial gastrocnemius than N mice. Furthermore, we hypothesized that augmented capillarization in MM would principally be a function of the smaller fibers in the superficial aspect of this muscle. On average, MM had higher capillary-to-fiber ratio and higher capillary density. Binning fibers according to size revealed that it was primarily the normal-sized fibers of the MM that had higher capillarity. The small fibers seen in the superficial region of MM were distinct from N mice in that they had heterogenous myofibrillar ATPase staining and patchy SDH staining in the interior of the fibers. These results support the hypothesis that the MM have higher indices of capillarity, caused primarily by greater capillary number around normally sized fibers. These alterations are consistent with the superior mass-specific aerobic function of these muscles.