This paper examines how much of the difference in the size of the informal sector and in per capita income across countries can be accounted by regulation costs and enforcement of financial contracts. It constructs and solves numerically a general equilibrium model with credit constrained heterogenous agents, occupational choices over formal and informal businesses, financial frictions and a government sector which imposes taxes and regulations on formal firms. The benefit from formalization is better access to outside finance. The quantitative exercises suggest that: (i) regulation costs and not the level of enforcement account for differences in the size of the informal sector between United States and Mediterranean Europe; (ii) for a developing country like Peru, however, contract enforcement and regulation costs are equally important in accounting for the size of the informal sector; and (iii) regulation costs and contract enforcement do not account for most of the income differences observed among countries.