Aquatic habitat is affected by paper mill effluent discharge in many ways. The effect of paper mill effluent on the gill ATPases was studied in freshwater fish Channa punctata (Bloch) exposed to 1%(v/v) of effluent for 15, 30, and 60 days. There was a time-dependent significant (P<0.05-0.001) decrease in all the ATPase activities measured, viz., total, Na(+), K(+)--and ouabain-insensitive ATPase in gill. ATPases play an important role in maintenance of functional integrity of plasma membrane and in several intracellular functions and are considered to be a sensitive indicator of toxicity. In addition to this, branchial ATPases are intimately involved in osmoregulation, acid-base regulation, and respiration of fish. The inhibition of ATPases in gills by, e.g., paper mill effluent could cause disruption of these processes. It is suggested that measurement of ATPases could also be used as a surrogate biomarker of exposure to chemical pollutants.