The gene encoding the fast skeletal isoform of the chick troponin I (sTnI) protein has been sequenced and its organization into exons and introns established. The gene is approximately 4.5 kb in length and composed of 8 exons, the first of which contains solely 5' untranslated sequence. In addition to its major mRNA product, there is evidence that the sTnI gene encodes a second mRNA, present at low abundance levels in embryonic skeletal muscle. Sl nuclease protection and primer extension experiments indicate that the low abundance mRNA is initiated approximately 47 nucleotides upstream of the major transcriptional initiation site. Both mRNAs appear to encode identical sTnI polypeptides. A comparison of nucleotide sequence in the 5' flanking region of several muscle-specific genes, including the sTnI gene, reveals a heptanucleotide consensus sequence, 5'-CATTCCT-3', which is conserved in the 5' flanking regions of many vertebrate contractile protein genes.