The murine homologue of the Drosophila twist gene has been shown to be essential for head mesenchyme formation and to act as an inhibitor of muscle differentiation. This paper presents a detailed analysis of M-twist expression patterns from day 7 post coitum (p.c.) to day 18 p.c., indicating a more general function of the M-twist gene. At day 7 p.c., M-twist is expressed in the mesoderm outside the primitive streak. Later M-twist message is predominantly found in the somites, the head mesenchyme, the branchial arches, the limbs, and in the mesenchyme underneath the epidermis. Beginning at day 8 p.c., M-twist is mainly expressed in undifferentiated cells committed to muscle and cartilage development: this expression is consistent with a suggested role of M-twist in inhibiting overt muscle and cartilage differentiation. However, during organogenesis, M-twist is expressed in several areas of mesenchyme-epithelia interactions, suggesting additional tissue specific functions.