The interest in the knowledge of the ecology of soft-sediment communities is increasing due to the importance that these environments pose to ecologists and environmentalists. However, the ecology of Argentinean sandy beaches is almost unknown, in spite of being relatively common landscapes. In this work we describe and compare the macroinfaunal assemblages in the two largest sandy beaches within the Monte León National Park, the first marine-coastal park of Argentina. Macrobenthic samples were collected and environmental descriptions were performed by quantifying and comparing the organic matter content, grain size distribution and depth of the substratum available for macroinfaunal colonization. Results show that polychaetes and crustaceans are the dominant taxa and that the two beaches differ in the physicochemical characteristics, suggesting that a relation between the benthic fauna and physicochemical factors exists. Although they appeared similar at the landscape scale in preliminary visual surveys, the geomorphological differences between habitats are likely to drive strong differences in the structure of the macroinfaunal assemblages. The monitoring of visible landscape features is likely to undermine the efficiency of protection and managing strategies if not complemented by geomorphological and ecological surveys of less visible ecosystem variables.