The ultrastructural pathological changes in the renal haemopoietic tissue of Channa punctatus (Bloch) experimentally infected with 2 species of aeromonads, Aeromonas hydrophila and Aeromonas salmonicida, were reported. Adult C. punctatus specimens were injected intramuscularly, once at a time, with a chosen sublethal dose (2 × 10 9 cfu mL -1) of aeromonads at 5 μL g -1 body weight of fish and were exposed to 72 h of duration. The leucocytes (especially macrophages and melanomacrophages) and other associated cells from the pronephric head kidney of aeromonadinfected C. punctatus were studied by fixed ultrathin sections for transmission electron microscopy. The sinusoidal cells, barrier cells, epithelioid cells, macrophages, and melanomacrophages in particular were infected by the chosen sublethal dose of aeromonads, and tissue destruction was initiated at this primary stage of infection. Other major changes were the increases in the number as well as the granular contents of macrophage and melanomacrophage cells for phagocytic activity against invading pathogens. © TÜBİTAK.