In a 3.8-kilobase mouse DNA sequence encoding amino acid sequences for the pro alpha 1(I) chain of type I procollagen, 14 coding sequences were identified which specify a sequence 95% homologous to amino acid residues 568 to 963 of the bovine alpha 1(I) chain. All of these coding sequences were flanked by appropriate splice junctions following the GT/AG rule. These observations suggest, but do not prove, that this pro alpha 1(I) gene is transcriptionally active. Of the 14 coding sequences, 7 were 54 base pairs in length, whereas the remainder were higher multiples of 54 base pairs. Nonrandom utilization of codons pertained throughout all of the coding sequences showing a preference (56%) for U in the wobble position. Two of the intervening sequences encoded imperfect vestiges of coding sequences which exhibited a codon preference different from that of the pro alpha 1(I) gene proper and were not flanked by splice junctions. One intervening sequence encoded a member of the mouse B1 family of middle repetitive sequences. It was flanked by 8-base-pair direct repeats and had a truncated A-rich region, suggesting that it may be a mobile element. Within this element were sequences which could function as a RNA polymerase III split promoter.