The distributions of oxidative capacities among type-identified muscle fibers in the developing cat diaphragm were examined by quantifying succinate dehydrogenase (SDH) activity using a microdensitometric procedure. Animals were studied during the first six weeks of postnatal development and compared to adults. Muscle fiber SDH activities were initially low during the first 2 postnatal wk, then increased to their highest values between 3 and 6 wk. Thereafter, fiber SDH activities declined to adult values. At each age, the distributions of SDH activities for both type I and II fibers were unimodal. Thus, no objective basis exists for subclassifying type II fibers based on differences in oxidative capacity. Fibers could be subclassified as type IIA, IIB, or IIC based on the acid pH lability of ATPase staining. In neonates, approximately 90% of all fibers were classified as type IIC. Thereafter, the proportion of IIC fibers decreased while the proportions of type I, IIA, and IIB increased. Adult fiber type proportions were reached by 6 wk of age. The SDH activity of type I fibers was generally higher than that of type II fibers at all ages, although there was considerable overlap in the distributions of SDH activities among type I and II fibers. The SDH activity of type IIC fibers was also higher than that of either type IIA or IIB during development. Only in the adult diaphragm was the SDH activity of type IIA fibers higher than that of type IIB. At no age could type IIA, IIB, or IIC fibers be discriminated based solely on differences in oxidative capacity.