Studies on the effect of monochrotophos (an organophosphate) on changes in bimodal oxygen consumption in an air breathing climbing, pereh Anabas testudineus (Bloch) have been made. Control group of fishes relies more on aquatic gas exchange obtaining 61. 5% of 02 from water where as only 38. 5% oxygen is obtained from air under normal bimodal condition. Exposure of fishes to sublethal concentration (15mg/l) of monohit caused decrease in total 02 uptake but increased aerial respiration as compared to control. Drop in aquatic respiration ranged from 45. 5-56. 0% while increase in aerial respiration ranged from 9. 6-68.9% as compared to control. The reasons for such changes in O2 uptake has been discussed in this paper.