OUfcomprehension ofthe molecular mechanisms underlying embryogenesis has been greatly enhanced by the identification and characterization of associated extracellular matrix macromolecules. Using Xenopus laevis as a model, we investigated the expression and distribution of SPARC (Secreted Protein, Acidic. Rich in Cysteine; also called osteonectin and BM-401 during early embryonic development. SPARC has been found to be enriched in tissues undergoing rapid morphological development, differentiation, and remodeling. In Xenopus. SPARCtranscripts are first expressed by primordial cells which give risetothefirst embryonic tissues, the notochord and somites. SPARC RNA levels remained high throughout the rapid morphological development and differentiation phase of these tissues, and then rapidly decreased. Of particular interest, SPARC protein began to accumulate within the intersomitic clefts atthe onset oftrunk myotome contraction. The intersomitic enrichment of SPARC remained high as long as the myotomes remained electrically coupled, principally by gap junctions. As myotomes became innervated, SPARC expression decreased dramatically within the somites. SPARCwas also found to be enriched within other tissues, such as the neural tube and epidermis. In addition, the selective spatial-temporal enrichment of SPARC suggests it makes important calcium-dependent contributions to early morphological development.