There are four myogenic factors of the MyoD family present in mammals, each have distinct patterns of expression during skeletal myogenesis in the mouse. This differs between axial (myotome) and peripheral (limb) muscle masses, which are derived from the medial and lateral parts of the somite, respectively. The onset of expression of the muscle structural genes also differs, suggesting differences in their activation threshold in response to different factors. In all cases myf-5 is the first gene to be expressed. In order to examine this more closely, we have introduced an nlsLacZ sequence into the myf-5 gene by homologous recombination. This has permitted us to follow expression of this gene at the cellular level. While the immediate precursors of the myotome are labelled, we have shown that peripheral muscle masses such as those in the limb are founded by muscle precursor cells which are initially myf-5 negative. At later stages of development the gene is transcribed in all skeletal muscles, irrespective of their origin and fibre type. We discuss the unexpected finding which was made with the myf-5/nlsLacZ mice, that myf-5 is also expressed in the central nervous system.