Among the first tissues to differentiate in the mammalian embryo are cardiac and subsequently skeletal striated muscle. We have developed specific cRNA probes corresponding to the 5′ noncoding regions of α-cardiac and α-skeletal actin mRNAs in order to investigate myogenesis in the mouse embryo. Transcripts coding for cardiac actin which is the major isoform of the adult heart can first be detected between 7·5 and 7·8 days p.c. in the developing heart and are observed in all somites as they are formed. In addition, α-skeletal actin transcripts are accumulated at much lower levels in cardiac tissue and newly formed somites; both heart and skeletal muscle show co expression of this actin gene pair at all stages of development examined. The fact that cardiac actin transcripts can be observed in the myotomal portion of the somite prior to muscle fibre differentiation indicates that cardiac actin transcripts (and to a lesser extent skeletal actin transcripts) are markers not only of striated muscle tissue, but also of earlier stages of the myogenic programme in vivo.