In vertebrates, hematopoietic and vascular progenitors develop from ventral mesoderm. The first primitive wave of hematopoiesis yields embryonic red blood cells, whereas progenitor cells of subsequent definitive waves form all hematopoietic cell lineages. In this report we examine the development of hematopoietic and vasculogenic cells in normal zebrafish and characterize defects inclocheandspadetailmutant embryos. The zebrafish homologs oflmo2, c-myb, fli1, flk1, andflt4have been cloned and characterized in this study. Expression of these genes identifies embryonic regions that contain hematopoietic and vascular progenitor cells. The expression ofc-mybalso identifies definitive hematopoietic cells in the ventral wall of the dorsal aorta. Analysis ofb316mutant embryos that carry a deletion of thec-mybgene demonstrates thatc-mybis not required for primitive erythropoiesis in zebrafish even though it is expressed in these cells. Bothclocheandspadetailmutant embryos have defects in primitive hematopoiesis and definitive hematopoiesis. Theclochemutants also have significant decreases in vascular gene expression, whereasspadetailmutants expressed normal levels of these genes. These studies demonstrate that the molecular mechanisms that regulate hematopoiesis and vasculogenesis have been conserved throughout vertebrate evolution and thecloandsptgenes are key regulators of these programs.