Acid rain is a great problem in our world. It causes fish and plants to die in our water. In this paper, a mathematical model is proposed to study the effect of acidic chemicals on fish population and also effects of remedial liming of lakes on it. Using stability theory of differential equations and computer simulation, it is shown that due to the effort (liming), acid concentration can be reduced to a desired level to save fisheries from extinction. © 2002 Elsevier Science Inc. All rights reserved.