The head-kidney (HK) is a unique immune, hemopoietic, and endocrine organ in fish. Twenty fish were used to study the structure, ultrastructure, and immunohistochemistry of the HK of Grass carp. The HK of grass carp was consisted mainly of a network of hematopoietic cells with a few kidney tubules. The HK contained all the developmental stages of erythrocytes and leucocytes, in addition to the presence of rodlet cells, dendritic cells, and epithelial reticular cells. Two populations of lymphocytes could be identified in HK; B-lymphocytes expressed CD20 and T cells expressed CD3. The cytotoxic T lymphocytes (CTLs) expressed CD8, while polymorphic granulocytes expressed iNOS2. Neutrophils showed myeloperoxidase immunoreactivity, as well as eosinophils expressed CCR3. Furthermore, macrophages were the most frequent cells found in the anterior kidney, which aggregated into structures called melanomacrophage centers (MMCs). Macrophages and monocytes showed immunoreactivity for CD68, CD163, and CD64. On the other hand, the interrenal cells (ICs) and chromaffin cells were mixed and arranged in large anastomosing cords intermingled with numerous capillary vessels that that lined the posterior cardinal veins of the head kidney. The cytoplasm of the interrenal cells showed the characteristic features of steroid-producing tissues. Three types of chromaffin cells; noradrenaline, adrenaline, and small granule containing cells could be distinguished in HK of grass carp. GFAP-positive sustentacular cells were identified for the first time among the chromaffin cells. The present study showed the presence of hemopoietic cells, MMCs, rodlet cells, immune cells, as well as one type of interrenal cells and three types of chromaffin cells in the HK of grass carp.