Krüppel-like transcription factors (Klfs), each of which contains a CACCC-box binding domain, have been investigated in a variety of developmental processes, such as angiogenesis, neurogenesis and somatic-cell reprogramming.However, the function and molecular mechanism by which the Klf family acts during developmental hematopoiesis remain elusive.Here, we report identification of 24 Klf family genes in zebrafish using bioinformatics.Gene expression profiling shows that 6 of these genes are expressed in blood and/or vascular endothelial cells during embryogenesis.Loss of function of 2 factors (klf3 or klf6a) leads to a decreased number of mature erythrocytes.Molecular studies indicate that both Klf3 and Klf6a are essential for erythroid cell differentiation and maturation but that these two proteins function in distinct manners.We find that Klt3 inhibits the expression of ferric-chelatereductase1b (frrs1b), thereby promoting the maturation of erythroid cells, whereas Klf6a controls the erythroid cell cycle by negatively regulating cdkn1a expression to determine the rate of red blood cell proliferation.Taken together, our study provides a global view of the Klf family members that contribute to hematopoiesis in zebrafish and sheds new light on the function and molecular mechanism by which Klf3 and Klf6a act during erythropoiesis invertebrates.