SM22α is a calponin-related protein that is expressed specifically in adult smooth muscle. To begin to define the mechanisms that regulate the establishment of the smooth muscle lineage, we analyzed the expression pattern of the SM22α gene during mouse embryogenesis. In situ hybridization demonstrated that SM22α transcripts were first expressed in vascular smooth muscle cells at about embryonic day (E) 9.5 and thereafter continued to be expressed in all smooth muscle cells into adulthood. In contrast to its smooth muscle specificity in adult tissues, SM22α was expressed transiently in the heart between E8.0 and E12.5 and in skeletal muscle cells in the myotomal compartment of the somites between E9.5 and E12.5. The expression of SM22α in smooth muscle cells, as well as early cardiac and skeletal muscle cells, suggests that there may be commonalities between the regulatory programs that direct muscle-specific gene expression in these three myogenic cell types.