The message for a second fibrinogen alpha chain has been cloned from a lamprey liver cDNA library. The sequence is unique in that the amino-terminal half is homologous to all other known alpha chains, including another from lamprey, but its carboxyl-terminal half is homologous to the carboxyl-terminal portions of beta and gamma chains, segments that compose the distal globular regions of fibrinogen. The structural pattern of this newly discovered alpha chain suggests that it could be a direct descendant of the archetypal chain that existed prior to the gene duplications that led to unique beta and gamma chains and before the dislocating events that gave rise to contemporary alpha chains.