Rat skeletal muscle contains up to four myosin isoforms. Analysis of single muscle fibers has identified type I, IIA, IID and IIB fibers containing type I, IIa, IId, and IIb myosin isoforms, respectively. Hybrid fibers have at least two myosin isoforms, such as myosin heavy chain (MHC) I+MHCIIa, MHCIIa+MHCIId or MHCIId+MHCIIb. These fiber types are identified as IC, IIC, IIAD, IIDA, IIDB or IIBD. The muscles of C57BL6J mice have not been extensively characterized, especially with regard to the presence and composition of hybrid fibers. The aim of this study was to examine the fiber composition of the soleus (SOL), extensor digitorum longus (EDL), tibialis anterior (TA) and gastrocnemius (GAS) muscles of C57BL6J mice using histochemical and biochemical methods. The SOL muscle contained type I fiber percentage, median ± semi amplitude, (37.42 ± 8.20%), IIA (38.62 ± 6.81%), IIAD (18.74 ± 6.95%) and IID (5.69 ± 3.09%) fibers. The EDL had type I (0.44 ± 1.27%), IC/IIC (3.05 ± 3.49%), IIA (0.46 ± 0.68%), IIAD (7.56 ± 4.51%), IID (0.46 ± 1.34%), IIDB (21.48 ± 7.33%) and IIB (66.01 ± 8.51%) fibers. The TA muscle also showed a predominance of type IIB (59.68 ± 9.95%) and IIDB (33.83 ± 15.85%) fibers followed by type IIAD (3.42 ± 4.87%), IID (2.25 ± 1.64%), IIA (1.12 ± 2.17%), I (0.17 ± 1.25%) and IC/IIC (0.008 ± 1.04%) fibers. The GAS muscle had type IIB (54.42 ± 8.11%), IIDB (19.37 ± 2.98%), IID (2.26 ± 2.24%), IIAD (12.40 ± 2.34%), IIA (5.73 ± 3.24%) and I (5.74 ± 2.55%) fibers. Thus, the SOL muscles of C57BL6J mice had predominantly type IIA fibers, unlike rats in which almost all of the fibers are type I. The EDL, TA and GAS muscles had predominantly IIB and IIDB fibers as in rats.