At day 10 in mouse gestation, the intraembryonic aorta-gonads-mesonephros (AGM) region generates the first definitive hematopoietic stem cells (HSCs) of the adult blood system. By 11 days postcoitum, the liver contains such HSCs. While HSCs of the adult bone marrow and late-stage fetal liver have been extensively characterized for cell surface markers, there has been no phenotypic description of the first HSCs during embryo development. We report here the temporal cell surface phenotype of HSCs from the AGM region and early fetal liver and show that all HSCs reside in the c-kit⁺ population. c-kit⁺ HSCs from AGM and liver are mainly CD34⁺ and in the AGM are in both Mac-1⁺ and Mac-1 fractions. These results demonstrate that during mouse ontogeny the first definitive HSCs are similar in cell surface phenotype to the HSCs of adult bone marrow but that spatial localization and developmental time are critical factors in the phenotypic assessment of this functional cell population.