Midkines compose a family of secreted, heparin-binding growth factors with neurotrophic activity in vitro, but largely unknown functions in mammals in vivo. Here we show that one member of this family, Midkine-b (Mdkb), is responsible for establishment of the neural plate border in zebrafish. We propose that MdkB acts downstream of several signaling factors, most notably retinoic acid, implicated in neural crest cell (ncc) induction and refines a zone of competence for ncc and Rohon-Beard (RB) sensory neuron formation. Overexpression of Mdkb expands the cell numbers of various ncc precursor subtypes and results in a significant increase in the number of RB neurons at the neural plate border. On the other hand, Morpholino-mediated knockdown of Mdkb leads to a dramatic reduction of ncc and a loss of sensory neurons. Our results imply that Mdkb is required for the earliest steps of cell specification at the neural plate border in zebrafish.