Actins are a small family of ubiquitous proteins that are essential cytoskeletal components and are highly conserved during evolution. Actins are usually divided into two classes, the cytoplasmic and muscle actins, which have different functional roles. Here we systematically analyzed the actin genes in the genome of the primitive chordate amphioxus ( Branchiostoma floridae
). We found that amphioxus contains more than 30 actin genes, many of which are linked. Phylogenetic analysis suggests the amphioxus actin genes have clearly undergone extensive expansion through tandem duplications. The actin genes' structure also varies a lot, containing 2 to 7 exons. We also cloned two muscle type of actin genes from the amphioxus (B. belcheri
) and compared their expression patterns during early development. The slight difference in their expression suggests functional diversification of these actin genes. Our results shed light on the evolution both of actin genes themselves and their functional roles in chordate development.