The present study analyzed the ultrastructural details of the immunopotentleucocytes from haemopoietic tissues of head kidney and relative distribution ofleucocytes from peripheral circulation of adult Heteropneustes fossilis (Bloch) to providea useful database for understanding its immunoefficiency. Leucocytes in the head kidneyof eight H. fossilis from wetlands near Kolkata, India, were studied through fixed thinsections for Transmission Electron Microscopy (TEM). Neutrophils, eosinophils andbasophils were identified among granulocytes based on their granular morphology andshape of the nucleus. Lymphocytes of the lymphoid series were also identifiable in H.fossilis. Macrophages were monocyte-derived cells having well developed pseudopodiain its surface and contained free ribosomes, lysosomes, RER, mitochondria,intracytoplasmic vacuoles and many cytoplasmic vesicles of variable size. The relativedistribution of leucocytes was also enumerated from differential counts. © 2014 by Nova Science Publishers, Inc. All rights reserved.