The haematology and leucocyte enzyme cytochemistry of Horabagrus brachysoma, a threatened freshwater catfish endemic to southern India, was studied using standard methods. Intra-specific variation was found for the haematological parameters, but this did not exceed the range of values observed in other catfishes. The relatively high haemoglobin (Hb) concentration may be indicative of an ability to breathe air and high activity. The erythrocytes are fully packed with Hb, revealing the bottom dwelling habit and primitive nature of this catfish. The leucocyte enzyme pattern also showed some variations from those of other fishes. Lymphocytes were positive only for peroxidase (PER) enzyme activity and negative for alkaline phosphatase (LAP), alpha-naphthyl acetate esterase (ANAE) and naphthol ASD chloroacetate esterase (ASDE). Monocytes were weakly positive for ANAE activity and negative for the other three enzymes tested. Neutrophils were negative for LAP, ANAE and ASDE but showed a moderately strong positive reaction for PER. Basophils and eosinophils were found to be devoid of all of these enzymes. Thrombocytes were observed to have weakly positive PER and ASDE, but there was no demonstrable LAP and ANAE activity. A number of characteristics were identified that distinguish this species from other fishes: (1) lymphocytes of H. brachysoma are actively engaged in both phagocytosis and defence mechanisms, while the monocytes participate in cellular defence mechanisms, primarily phagocytosis; (2) thrombocytes function as a protection barrier as well as carrying out their normal function of haemato plug formation during blood clotting. Results from the haematological and leucocyte cytochemical analyses reveal the haematological make-up and effective immune mechanism of this threatened fish and show it to be highly adaptive in nature. The data may be useful in programmes aiming the effective conservation of this species.