Using recombinant DNA techniques, DNA fragments coding for bovine prothrombin mRNA have been cloned and characterized. Structural studies have revealed that prothrombin mRNA encodes a precursor protein having an amino-terminal extension of 43 amino acid residues. Using bovine prothrombin cDNA as a hybridization probe, the genes coding for human and bovine prothrombin have been isolated and partially characterized. The organization of the prothrombin gene is similar, but not identical to the organization of the genes coding for the other vitamin-K-dependent clotting factors.