The full-length sequence and developmental expression of an amphioxus Wnt gene (AmphiWnt11) are described. A phylogenetic analysis of all known full-length Wnt11 sequences indicates that a gene duplication occurred at the base of the vertebrate Wnt11 clade. The developmental expression domains of AmphiWnt11 resemble those of Wnt11 homologs in vertebrates. The earliest detectable expression is transiently associated with the dorsal lip of the blastopore. At the neurula stage, AmphiWnt11 is expressed in myotomal muscle cells; however, AmphiWnt11 transcription is not associated with metameric pre-patterning prior to morphological segmentation. Finally, in amphioxus and the vertebrates, Wnt11 homologs are expressed in anteroventral ectoderm and in association with the tailbud and the tail fin. Thus, in amphioxus and lower vertebrates, the posterior expression of Wnt11 may be involved in tail fin outgrowth, and this ancient genetic program might have been co-opted at least in part for lateral appendage development during vertebrate evolution. genesis 27:1-5, 2000.