Studies on the effect of Metasystox (an organophosphate pesticide) on changes in certain haematological indices were made in a fresh water air breathing siluroid fish, Mystus tengara (Ham.). In control group of fishes the RBC counts, Hb content, PCV, MCV and MCHC were respectively 2.26±0.06 3 3 million/mm , 11.78±0.13%, 25.45±1.5%, 113.32? m , 52.45 pg and 49.57%. The acute exposure of Metasystox for 96 hrs at sublethal dose (5.19 mg/l) caused reduction in RBC counts, Hb content, PCV, MCV, MCH and MCHC as compared to control. The reasons for such changes have been discussed in this paper.