Activation of prothrombin to thrombin is the key reaction in blood coagulation cascade. We have recently shown that Australian rough scaled snake, Tropidechis carinatus, possesses two parallel prothrombin activator systems. Trocarin D, a venom prothrombin activator produced in the venom gland, plays an offensive role as a toxin, whereas factor X is produced in the liver and plays a role in the hemostatic mechanism. These two proteins are structurally similar and have identical domain architecture. Because of the differences in their physiological roles, and tissue-specific expression, we determined the gene structure of these closely related proteins. Both the genes have eight exons similar to all mammalian factor X genes. All the exon-intron boundaries of these two genes are at the same position and the splice junctions are almost identical. Partial sequencing of the introns shows that they share a very high degree of sequence identity indicating that the gene duplication is a recent event. Further studies on the characterization of these two genes particularly the promoter regions are in progress.