Janus kinase 2 (Jak2) transduces signals from hematopoietic cytokines and a gain-of-function mutation (Jak2(V617F)) is associated with myeloproliferative diseases particularly polycythemia vera. In this study, we examined the role of jak2a in zebrafish embryos in knock-down and over-expression studies using morpholinos (MOs) targeting the 5'untranslated region (UTR) (jak2a(UTR)-MO) and splice-site junction (jak2a(SS)-MO) of jak2a, a Jak inhibitor AG490 and a constitutive-active form of jak2a (jak2a(ca)). At 18 and 24 hours-post-fertilization (hpf), jak2a is expressed predominantly in the intermediate cell mass (ICM, site of primitive hematopoiesis) of wild-type and chordin morphant embryos (characterized by expansion of ICM). Both jak2a MOs and AG490 reduced gata1(+) (erythroid) cells in Tg(gata1:GFP) embryos, stat5 phosphorylation and gene expression associated with early progenitors (scl and lmo2), erythroid (gata1, alphahe1 and betahe1) and myeloid [spi1 (early) and mpo (late)] lineages. The chordin morphant is associated with increased stat5 phosphorylation and both jak2a MOs and treatment with AG490 significantly ameliorated ICM expansion and hematopoietic gene up-regulation in these embryos. Injection of plasmid encoding jak2a(ca) significantly increased erythropoiesis and expression of gata1, alphahe1 and betahe1, spi1, mpo and l-plastin. In conclusion, zebrafish jak2a is involved in primitive hematopoiesis under normal and deregulated conditions.