It is widely believed that the existence of ‘informal sector’ enterprises that visibly do not pay direct taxes reduces the willingness of owners of formal, tax-registered enterprises to pay their own taxes. We call this the adverse evasion spillover hypothesis. It is for several reasons hard to test this hypothesis, especially in this most general form. We test a more focused version, with two components. One is that the levels of tax compliance of formal firms are reduced when those firms perceive that they are adversely affected by direct economic competition from informal enterprises. The other is that these effects are especially marked for smaller formal sector firms. Two particular procedures enabled us to collect the data needed to test these hypotheses in a satisfactory way.