Swine skeletal muscle fibers have traditionally been classified as types I, IIA, IIB and their hybrid forms by determining the activity of myosin ATPase (mATPase). More recently, molecular, electrophoretic and immunohistochemical approaches have allowed the identification of IIx isoform-containing fibers. In this study, myosin isoforms present in different types of muscle fibers were identified by combining histochemical and immunohistochemical methods. Samples from the M. longissimus dorsi were collected from primiparus pigs (mean body weight, 150 Kg) 45 minutes after slaughtering and subsequently frozen in dry ice-cooled acetone. Serial cross sections were processed for mATPase and NADH-TR activities and for immunohistochemistry. Five histochemical fiber groups were identified: type I (dark), type IIA (light) and three intermediate types. A panel of monoclonal antibodies identified the slow ßI, IIa, IIx and IIb isoforms present in types I, IIA, IIX and IIB fibers and the hybrid group IIAX