This paper explores issues of international cooperation in CO, emission control, We consider three cases: ＇selfish,＇ in which region 1 controls optimally given its own damage costs and region 2 does not control; ＇altruistic,) in which region 1 controls optimally given total worldwide damage costs while region 2 does not control; and ＇optimal,＇ in which regions 1 and 2 exercise jointly optimal control of emissions and region 1 pays region 2 to participate, We find that neither region gains much in the selfish case; but both regions are better off in the optimal case than in the altruistic case, though region 1＇s benefits are negative in both these cases.