Specific probes were used to examine the accumulation of muscle actin mRNA during embryonic development of the ascidian Styela. Clones of a muscle actin gene were obtained from an adult mantle cDNA library. Four lines of evidence indicate that these clones correspond to a muscle actin gene. First, their coding regions share 11 of 14 diagnostic amino acid positions with mammalian smooth and skeletal muscle actins. Second, subclones that contain only the 3' noncoding region of the gene select mRNA coding for muscle actin, while subclones that include the coding region of the gene select mRNA coding for muscle and nonmuscle actins. Third, a probe that contains only the 3' noncoding region detects a single band, corresponding to a 2 kb transcript, while a probe that includes the coding region detects the 2 kb transcript and at least one other band, presumably a cytoplasmic actin transcript. Fourth, the 3' noncoding region probe detects transcripts only in muscle cells and their precursors, while the coding region probe detects transcripts in muscle and nonmuscle cells. The muscle actin transcript is present at very low levels in eggs and early embryos, begins to accumulate between the early gastrula and tailbud stages, and by the tadpole stage attains a level about 25-fold higher than in the egg. In situ hybridization showed that embryonic muscle actin transcripts are restricted to the muscle cell lineages. These transcripts were initially observed in primary muscle lineage cells (descendants of the B4.1 blastomeres) at the early gastrula stage and continued to be present in these cells throughout embryonic development. In contrast, muscle actin transcripts did not appear in secondary muscle lineage cells (descendants of b4.2 and A4.1 blastomeres) until the mid-tailbud stage, and were not detected in mesenchyme cells, the presumptive adult muscle cell precursors, at any time during embryonic development. The results suggest that muscle actin gene expression is subject to spatial and temporal regulation in the muscle cell lineages.