This paper presents a procedure used to estimate the recreational use value of “Tablas de Daimiel” National Park by applying the Contingent Valuation Method (CVM) with single dichotomous choice, using the truncated mean as welfare measure. This valuation is based on 433 interviews of visitors made during the spring and summer 1996. The sample allocation among the bids offered in the surveys is based on the Duffield and Patterson’s method which allows to get the minimal variance of the truncated mean estimator. The recreational values of the Park are not significantly different for the two periods in which the surveys have been carried out. Finally, the tests to study the “interviewers effect” and to analyse the relations between the willingness to pay (WTP) and various interviewees’ features don’t invalidate the procedure adopted to determine the recreational use value of this National Park.