Abstract The stem cell leukemia (SCL) gene encodes a bHLH transcription factor essential to the primitive and definitive hematopoietic systems in vertebrates. In the zebrafish, studies of hematopoiesis have shown significant functional conservation with mammals. In order to assess the functions of scl in zebrafish primitive hematopoiesis, zebrafish embryos were analyzed after ablation of Scl protein by morpholino-modified antisense RNA injection. Embryos injected with morpholinos targeting the scl translation initiation site or two independent splice donor sites lacked circulating blood and exhibited cardiac edema by 28 hours post fertilization. RNA in situ hybridization analysis with a panel of hematopoietic-specific genes showed that myeloid lineage development in the anterior lateral mesoderm was absent upstream of draculin and pu.1 expression. In contrast, erythroid development in the posterior lateral mesoderm occurred with little or no functional Scl, as shown by expression of draculin, pu.1, and gata1 in this PLM region. Primitive erythroid cells failed to mature, as evidenced by the absence of circulating cells after 24 hpf. Only upon titration of scl morpholinos to near toxic levels was gata1 reduced or absent in embryos at 21 somites. These findings show that scl plays a vital role in development of all hematopoietic lineages in zebrafish. However, these data reveal differential effects of reduction or loss of Scl on hematopoietic development in the anterior lateral mesoderm versus the posterior lateral mesoderm. The results reveal that reduction or loss of Scl affects erythroid development in the posterior lateral mesoderm or ICM at a progenitor stage of erythroid development, after initiation of gata1 and embryonic globin expression. These data suggest that early erythroid development in the zebrafish requires little or no Scl, while maturation of erythroid progenitors cannot occur in the absence of Scl. Myeloid development in the anterior lateral mesoderm is dependent upon Scl. This embryonic analysis in the zebrafish is the first to demonstrate a transient, in vivo population of erythroid precursors in embryos with severely reduced or absent expression of Scl.