The zebrafish striped pattern results from the interplay among three pigment cell types; black melanophores, yellow xanthophores and silvery iridophores, making it a valuable model to study pattern formation in vivo. It has been suggested that iridophore proliferation, dispersal and cell shape transitions play an important role during stripe formation; however, the underlying molecular mechanisms remain poorly understood. Using gain- and loss-of-function alleles ofleucocyte tyrosine kinase(ltk) and a pharmacological inhibitor approach, we show that Ltk specifically regulates iridophore establishment, proliferation and survival. Mutants inshady/ltklack iridophores and display an abnormal body stripe pattern.Moonstonemutants, ltkmne, display ectopic iridophores, suggesting hyperactivity of the mutant Ltk. The dominantltkmneallele carries a missense mutation in a conserved position of the kinase domain that highly correlates with neuroblastomas in mammals. Chimeric analysis suggests a novel physiological role of Ltk in the regulation of iridophore proliferation by homotypic competition.