Forty-two kilobase pairs of cloned chicken DNA containing 80% of the pro alpha 2 (type I) collagen gene and 8 kilobase pairs of 3' flanking sequences have been isolated. Detailed analysis of these clones indicates that this collagen gene spans approximately 40 kilobase pairs of DNA and contains on the order of 50 introns. The fine structure of 40% of the pro alpha 2 gene, including its 3' end, was determined by Southern blot restriction endonuclease mapping using a 2.6-kilobase pair procollagen cDNA clone pCg45, as a probe, and by DNA sequence determination of more than 2 kilobase pairs of this part of the genome. Exons in the triple-helical coding region are all multiples of the 9 base pairs coding for the Gly-X-Y triplet and vary in size from 45 to 108 base paris. The sequences of all six exons in a 3.8-kilobase pair EcoRI fragment were determined. One of these, a 249-base pair exon, joins the collagen domains; it codes for the last 15 amino acids of the triple-helical coding region, the telopeptide, and the first 53 amino acids of the carboxy-terminal propeptide.