Effect of sublethal concentrations of Monocrotophos (4.5 ppm, 6.7 ppm and 13.5 ppm) on haematological parameters were studied in fresh water catfish Mystus vittatus. Monocrotophos toxicity resulted in a significant decrease in all the sub lethal concentrations and maximum at 13.5 ppm (RBC (-47), Hemoglobin content (-77), ESR(-75) and PCV % (-68) and a significant increase in WBC count (+55).