The acute and sublethal toxicity of Nuvan (Dichlorvos 76% E.C. – an organophosphate pesticide) on a commercial freshwater air breathing climbing perch, Anabas testudineus (Bloch) was evaluated under static bio-assay condition for 24, 48, 72 and 96 hrs. The LC„ (TLm) values were computed to be 30.65, 27.85, 23.70 and 18.50 ppm at water temperature 29.0±1.0°C. The safe concentration value was calculated to be 6.90 ppm at this temperature. The behavioural response of fish towards the toxicant (Nuvan) was grossly dependent on concentration and length of exposure. The mean values of opercular movements (per minute) gradually decreased with the increasing concentration of Nuvan. The reason for such changes have been discussed in the present paper.