Histochemical techniques based upon the pH sensitivity of myofibrillar adenosine triphosphatase are used to differentiate avian skeletal muscle fibers as either fast or slow. Pars thoracica m. pectoralis (PM) of several avian species, Pars cranialis m. latissimi dorsi (LDCR) of the Japanese quail, and M. tensor propatagialis (TP) of the domestic pigeon are examined. Fast fibers predominate in the PM, and slow fibers in the LDCR. The TP shows marked internal variation in the distribution of muscle fibers. The occurrence of fast and slow muscle fibers, both intra- and inter-muscularly, is correlated with their functional adaptations.