In this paper a mathematical model is proposed and analysed to study the effect of an environmental pollutant on two interacting biological species. The interaction between the two species is considered to be of three types, namely, competition, cooperation, and prey᎐predator. In each case criteria for local stability, instability, and global stability of the nonnegative equilibria of the system are obtained. The effect of diffusion on the equilibrium state of the system is also studied. ᮊ 2000 Academic Press