Somites are the source of hypaxial musculature including skeletal muscles of the limb, tongue, and trunk. To get insight into the function of mouse Lbx1 homeobox gene in early somitic mesoderm differentiation, in situ hybridization analyses were performed. At the 4-6 somite stage (8 dpc), Lbx1 was first expressed in the lateral portion of the epithelial somite and dermomyotomal epithelium. This was in contrast to the expression of myf-5 in the medial region of the somite. The lateral expression of Lbx1 in somitic mesoderm then occurred regionally along the anterior-posterior body axis. Later, at 10 dpc (stage 1 of limb bud development), Lbx1-positive migrating cells originated in the lateral dermomyotomal lips at occipital, forelimb, and hindlimb levels. They also expressed Pax-3 and c-met, known as markers of the migrating limb muscle precursor cells. In stage 4 hindlimb bud (11.5 dpc), the dorsal and ventral muscle precursor populations expressed Lbx1. In stage 8 forelimb buds (12.5 dpc), Lbx1 expression was reduced in the proximal muscle masses, where the high expression of myogenin accompanying muscle differentiation was detected. These results suggest that mouse Lbx1 might be involved in the commitment or determination of a muscle cell subpopulation during hypaxial musculature development.