The cytokine Macrophage Stimulating 1 (MST1/MSP/Hepatocyte Growth Factor-Like) is a ligand of the Met-related MST1-Receptor (MST1R/RON). Although MST1-deficient mice are viable, MST1R is essential in mice before gastrulation for implantation, and is a known oncogene in man. Here I report the identification, sequence, chromosomal location and embryonic expression of a novel zebrafish orthologue, termed macrophage stimulating 1 (mst1). mst1 shows a striking restriction of expression to the dorsal side of the embryo prior to gastrulation, and as gastrulation and somitogenesis proceed is expressed sequentially in the presumptive neurectoderm, the notochord, the somites, endodermal cells and in the syncytial yolk. This dynamic pattern is largely conserved in tetrapod vertebrates, suggesting that the appearance of MST1, may have played an early role in the evolution of the vertebrate body plan.