SM22α is a 22-kDa smooth muscle cell (SMC) lineage-restricted protein that physically associates with cytoskeletal actin filament bundles in contractile SMCs. To examine the function of SM22α, gene targeting was used to generate SM22α-deficient (SM22^−/−LacZ) mice. The gene targeting strategy employed resulted in insertion of the bacterial lacZ reporter gene at the SM22α initiation codon, permitting precise analysis of the temporal and spatial pattern of SM22α transcriptional activation in the developing mouse. Northern and Western blot analyses confirmed that the gene targeting strategy resulted in a null mutation. Histological analysis of SM22^+/−LacZ embryos revealed detectable β-galactosidase activity in the unturned embryonic day 8.0 embryo in the layer of cells surrounding the paired dorsal aortae concomitant with its expression in the primitive heart tube, cephalic mesenchyme, and yolk sac vasculature. Subsequently, during postnatal development, β-galactosidase activity was observed exclusively in arterial, venous, and visceral SMCs. SM22α-deficient mice are viable and fertile. Their blood pressure and heart rate do not differ significantly from their control SM22α^+/− and SM22α^+/+ littermates. The vasculature and SMC-containing tissues of SM22α-deficient mice develop normally and appear to be histologically and ultrastructurally similar to those of their control littermates. Taken together, these data demonstrate that SM22α is not required for basal homeostatic functions mediated by vascular and visceral SMCs in the developing mouse. These data also suggest that signaling pathways that regulate SMC specification and differentiation from local mesenchyme are activated earlier in the angiogenic program than previously recognized.