Oxygen uptake (VO2) during graded hypoxia, rate of hypoxia acclimation, breathing frequency (fR), breath volume (VS, R) and gill ventilation (VG) were measured in Hoplias malabaricus. Normoxia and hypoxia acclimated fish had similar and constant VO2 and VG in a range of water PO2 from 150 to 25 mmHg. Hypoxia acclimated fish showed significantly higher VO2 in severe hypoxia (PO2 <15 mmHg). Normoxia acclimated fish showed symptoms similar to hypoxic coma after 1 h of exposure to water PO2 of 10 mmHg whereas the same symptoms were observed only at PO2 of 5 mmHg for fish acclimated to hypoxia. Fish required 14 days to achieve full acclimation to hypoxia (PO2 ≥25 mmHg). Lowering of water PO2 from 150 to 25 mmHg resulted in normoxic fish showing a 3–2 fold increase in VG. The increase was the result of an elevation in VS, R rather than fR. Among normoxia acclimated specimens, small fish showed a higher VG per unit weight than the large ones in both normoxia (PO2 =150 mmHg) and hypoxia (PO2 = 15 mmHg). A decrease in the ventilatory requirement (VG/VO2) with increased body weight was recorded in hypoxia (PO2 = 15 mmHg).