In developing countries, the rapid proliferation of informal firms – low-technology unlicensed micro-enterprises – is having significant environmental impacts. Yet environmental management authorities typically ignore such firms. This paper estimates the annual net benefits (benefits minus costs) of controlling particulate emissions from a collection of informal brick kilns in Ciudad Juárez, Mexico and from two of the city's leading formal industrial polluters. We find that the annual net benefits of controlling brick kiln emissions are substantial – in the tens of millions of dollars – and exceed those for the two formal industrial facilities by a significant margin. These results suggest that, in some cases, the conventional allocation of pollution control resources across formal and informal polluters may be suboptimal.