Factor X is one of six vitamin K dependent proteins known to be involved in blood coagulation, the others being factor VII, factor IX, prothrombin, protein S, and protein C. In the present studies, recombinant bacteriophage containing overlapping DNA inserts coding for the gene for human factor X have been isolated and characterized. These DNA inserts code for almost the entire gene for factor X, extending from the prepro leader peptide through the 3' noncoding region of the transcription product. The organization of the gene for factor X was established by DNA sequencing to identify the location of the introns and exons in the gene. Seven introns and eight exons were identified and their intron/exon boundaries established. The seven introns interrupt the coding sequence at essentially identical locations in the amino acid sequence as the introns in the genes for human factor IX and protein C. In addition, the introns in the gene for factor X divide the coding sequence into discrete exons that code for potential structural and functional domains of the protein. This information provides strong evidence to support the suggestion that the vitamin K dependent proteins present in plasma have evolved from a single, common gene and that this ancestral gene arose through a process that involved the assembly of small protein coding units of DNA into a single gene.