This paper investigates the effect of working in the underground sector on the level of individual underground expenditures. We show that it can be decomposed into a network effect, a non-separability effect and an income effect. Our empirical analysis uses micro data from a randomized survey conducted in the province of Québec for 1993. GMM methods allow us to take into account censoring and measurement error on the dependent variable as well as the possible endogeneity of the conditioning labor supply variables. We find that an increase in the underground labor supply has a strong positive effect on the level of underground expenditures. This relationship amplifies the impact of tax policy and of the enforcement system on the size of the underground economy.