All mature blood cell types in the adult animal arise from hematopoietic stem cells (HSCs). The developmental cues regulating HSC ontogeny are incompletely understood, as is the role of Wnt/b-catenin signaling in the process. Using zebrafish, we have demonstrated that Wnt signaling is required to direct an amplification of nascent HSCs in the aorta. Wnt9a is specifically required for this process, and cannot be replaced by other canonical Wnt ligands. This is unusual for Wnt signaling, and we have now identified the receptor on HSC precursors that underlies this specificity. Our results support a general paradigm where early signaling events, including highly specific Wnt interactions, prime HSCs for later developmental processes.