Vertebrate limb regeneration occurs in anamniotes such as newts, salamanders, and zebrafish. After appendage amputation, the resection site is covered by a wound epidermis capping the underlying mature tissues of the stump from which the blastema emerges. The blastema is a mass of progenitor cells that constitute an apical growth zone. During outgrowth formation, the proximal blastemal cells progressively leave the zone and undergo the differentiation that results in the replacement of the amputated structures. Little is known about the mechanisms triggering regenerative events after injury. The zebrafish caudal fin provides a valuable model to study the mechanisms of regeneration. Zebrafish blastemal cells express specific genes, such as the homeobox-containing transcription factors msxB and msxC, and secreted signal FGF20a. In this study, we set out to identify signals that are transcriptionally upregulated after fin amputation and before blastema formation. Accordingly, a gene encoding a TGFbeta-related ligand, activin-betaA (actbetaA), was found to be strongly induced within 6 hr after fin amputation at the wound margin, and later in the blastema. Inhibition of Activin signaling through two specific chemical inhibitors, SB431542 and SB505124, lead to the early and complete block of regeneration. The morpholino knockdown of actbetaA and its receptor alk4 impaired the progression of regeneration. Closer examination of the phenotype revealed that Activin signaling is necessary for cell migration during wound healing and blastemal proliferation. These findings reveal a role of Activin-betaA signaling in the tissue repair after injury and subsequent outgrowth formation during epigenetic regeneration of the vertebrate appendage.