In developing countries, urban clusters of manufacturers which are "informal"—small-scale, unlicensed and virtually unregulated—can have severe environmental impacts. Yet pollution control efforts have traditionally focused on large industrial sources, in part because the problem is not well-understood. This paper presents a benefit-cost analysis of four practical strategies for reducing emissions from traditional brick kilns in Ciudad Juárez, Mexico. To our knowledge, it is the first such analysis of informal sources. We find very significant net benefits for three of the four control strategies. These results suggest that informal polluters should be a high priority for environmental regulators.