Haemopoietic tissues of the head kidney of Clarias batrachus were studied for morphometric analysis and ultrastructural descriptions of the erythropoietic and leucopoietic cell populations. Eight successive steps of developing erythrocytes from small lymphoid haemoblasts (SLHs) to mature erythrocytes (MEs) were distinguished. Progressive loss of RNA and synthesis of haemoglobin during haemopoiesis were coupled with denser nucleus and changes in cellular shapes. Transmission electron microscopy aided in clear distinction between cell lineages in the development of erythrocytes, granulocytes, and lymphocytes. Ultrastructural analysis of the pronephric stromal cells forming adherent multilayer of haemopoietic microenvironment was explicit in describing their ability to promote haemopoiesis in fish. Barrier cells observed within the haemopoietic tissue indicate their functional significance in the haematopoietic microenvironment. © TÜBİTAK.