Granulocyte colony stimulating factor (G-CSF) has been shown in mammalia to activate a series of signal transduction systems and exert various biological effects, such as controlling the differentiation, proliferation, and survival of granulocytes, promoting the movement of hematopoietic stem cells from the bone marrow to the bloodstream, and triggering the development of T cells, dendritic cells, and immune tolerance in transplants. In this study, the mRNA of flounder G-CSF (PoG-CSF) and its receptor (PoGCSFR) were detected and widely expressed in all examined tissues with the highest expression in peritoneal cells. G-CSF