The complete nucleotide sequences of two chromosomally linked actin genes from the sea urchin Strongylocentrotus franciscanus are presented. The genes are separated by 5.7 kilobases, occur in the same transcriptional orientation, and contain introns in identical positions. The structures and nucleotide sequences of the two genes are extremely similar, suggesting that they arose through a recent duplication. Comparison of the nucleotide sequences of the genes allows inferences to be made about mutational mechanisms active since the duplication event. Whereas point mutations predominate in the coding regions, the introns and flanking DNA are more heavily influenced by a variety of events that cause simultaneous changes in short regions of DNA.