I model the interaction between a regulator and polluting firms as a Stackelberg differential game in which the regulator leads. The firms create pollution, which results in a stock externality. I analyze the intertemporal effects of alternate pollution control measures in a competitive industry. The principal issue here concerns the dynamic inconsistency of the optimal solution. Inter alia, I compare the steady state levels of pollution under optimal and under time consistent policies. Forthcoming in Environmental and Resource Economics