The ATPase reaction and its pH lability demonstrate three fiber types in the adult mouse gastrocnemius; Type I (light staining with alkaline preincubation and dark with acid preincubation), Type IIA (dark after alkaline preincubation), type IIB (dark after alkaline and acid preincubation). the SDH and NADH‐tetrazolium reductase reactions also demonstrate three types of fibers; those low (A), intermediate (B) or high (C) in oxidative enzyme activity. However, the use of both procedures in serial sections demonstrates that four different combinations occur; the IIB fibers are high in SDH activity, the I fibers are intermediate in SDH enzyme activity, while the IIA fibers are either low or intermediate in SDH activity. These fiber types are present within the gastrocnemius muscle in a distinct pattern of zones. the predominant fiber type, located in the superficial half of the muscle, is the IIA (A) fiber which is high in ATPase and low in SDH activity. This is consistent with the fact that the gastrocnemius is generally considered a white, fast muscle. the IIB and I fibers are fewer in number and are located deeper in the muscle.