This study characterized histochemically three fast fiber types (IIB, IID, IIA) in skeletal muscles of mouse, rat, and rabbit, with special reference to fiber types IIB and IID. The results are complemented by biochemical analyses of myosin heavy chain composition in these muscles. Fiber type delineation is based on various methods for mATPase staining with pre-incubations and assays under different conditions. In rat and mouse, IIB and IID fibers can be best distinguished according to their different mATPase stabilities towards formaldehyde and alkaline pH. In rabbit, the method of Matoba and Gollnick using acid pre-incubation provided best and most reproducible results. In addition to their different mATPase stabilities, the three fast fiber types differ with regard to their oxidative capacities and cross-sectional fiber areas in the three species. In general, Type IIB fibers are the largest and least oxidative, Type IIA fibers the smallest and most oxidative, and Type IID fibers intermediate. In rabbit, Type IID fibers are the predominant fast fiber population in extensor digitorum longus, psoas, and tibialis anterior muscles. As judged from histochemistry, these muscles of rabbit do not contain pure Type IIB fibers. This is in accordance with biochemical results that show the HCIId to form the majority of the myosin heavy chain complement expressed in these muscles. On the other hand, IIB fibers are numerous in rabbit adductor magnus, gastrocnemius, and vastus lateralis muscles. Similarly, appreciable amounts of myosin heavy chain HCIIb are found in the three latter muscles of rabbit.