This article presents an agent-based computational analysis of the effects of externality zoning on environmental justice (EJ). We experiment with two ideal types of externality zoning: proactive and reactive. In the absence of zoning, environmental injustice emerges and minority agents have lower average environmental quality than majority agents. With proactive zoning, which allows polluting firms only in designated zones, EJ problems are less severe and appear more tractable. With reactive zoning, which creates buffering zones around polluting firms, environmental injustice tends to emerge more quickly as compared with proactive zoning but tends to decline over time. This analysis examines a possible policy tool available for cities to ameliorate environmental injustice.