We examine the effects of entrance .fees and other factors in visitation to U.S. national park areas under two partially competing hypotheses: 1) fees are significant explanatory variables; and 2) individual area attributes are the primary determinants of visitation rates. (National park areas include natural protected areas, historic areas, and other categories in the national park system). We find that areas comprising natural protected areas behaved like economic substitutes for each other, and historic areas like economic complements. In addition, the results have confirmed the importance of individual park attributes in visitation, but are equivocal on the role of entrance fees. The role of other socio-economic variables and of park size is also analyzed.