The deterioration of protected natural areas due to over-visiting is a frequent situation. In this article we modelize the management of access to such places in an adverse selection setting. The regulator offers each visitor a contract based on a limited right of access together with a monetary transfer; both right and transfer depend on the personal valuation of the natural place by each visitor. The aggregation of the limited rights of access respects the congestion capacity of the natural area. The visitors then have two possibilities: to decide the length of their visit or to reduce it in exchange for a green tax deduction. In both cases none of the visitors is denied access to the protected natural area, even when some visitors refuse the contract. The regulation cost can be financed by a price of entry or by a public budget. In the first case, the income effect of the contracts has to be taken into account, while in the second case a paradoxical question can emerge: how much taxpayers are willing to finance the conservation of protected natural areas that they consider places of exceptional interest.

Key words: ecotasas, ecotaxes, adverse selection, contracts, commons, environment, exclusion, incentives.
JEL classification: Q20.