The paper deals with the effect of pesticide on the pest affecting a paddy field and also on the fishes living in the soil water. An objective function is formed in terms of projected loss of individual fish, the projected profit of killing a pest and the cost of spraying insecticide for a finite time period. Next the corresponding pest management model is formed by considering the growth equations of paddy, pest, and fish of the soil water. The model is analyzed by control theoretic means to determine the threshold limits of pesticide to be used. Lastly, the optimal value of pesticide is obtained. It is shown that the optimal value lies within the threshold limits. Finally corresponding to this optimal threshold value of pesticide, the optimal values of the biomass of paddy and that of the fish of the soil water are also obtained. The results are finally verified numerically by special choice of parameters compatible with the model. © World Scientific Publishing Company.