The paper deals with the pesticide monocrotophos (0-600 ppm) induced changes in physiological and biochemical parameters related to growth and defense system in Azolla filiculoides. Monocrotophos treatment on fresh weight, dry weight, doubling time, chlorophyll, carotenoids, relative growth rate, lipid peroxidation, electrolyte leakage, proline accumulation and activities of superoxide dismutase and peroxidase were analyzed. Growth and photosynthetic pigments, i.e. chlorophyll and carotenoids were adversely affected by monocrotophos treatment and the inhibition was found to be dose dependent. Pesticide treatment with increasing doses accelerated the formation of reactive oxygen species, i.e. O-2 and H2O2 in cells progressively, whereby an enhanced lipid peroxidation and electrolyte leakage was noticed in A. filiculoides. Proline accumulation also showed a similar trend. As a consequence of reactive oxygen species (ROS) generation in monocrotophos treated plants, the activity of superoxide dismutase (SOD) and peroxidase (POD) were enhanced considerably.