The apical ectodermal ridge (AER) is indispensable for vertebrate limb development and requires Wnt/β-catenin signaling for induction and maintenance. We report identification and involvement of Wnt10a in AER formation during chick limb development. Chicken Wnt10a has 82% identity with mouse Wnt10a in the amino acid sequence. The Wnt10a gene was expressed broadly in the surface ectoderm from as early as stage 10. By stage 15, the expression was restricted to the surface ectoderm overlying the lateral plate mesoderm. Wnt10a expression became intensified in the presumptive limb ectoderm during AER formation, and subsequently intense expression signals persisted in the AER. Wnt10a misexpression led to ectopic Fgf8 expression in the developing limb ectoderm and induced translocation of β-catenin in chick embryo fibroblasts. These results suggest that Wnt10a is involved in AER formation in the chick limb bud through the Wnt/β-catenin signaling pathway. Developmental Dynamics 233:282–287, 2005. © 2005 Wiley-Liss, Inc.