The hilltopping behavior of individuals in a closed demographic unit of Euphydryas editha is examined. Both males and females hilltop but the behavior differs between the sexes. Most males appear to hilltop throughout their reproductive lives. Females hilltop only as virgins. The occurrence of hilltopping in a population is influenced by the density and age structure of the population. In addition some males appear not to hilltop but instead patrol" below the hilltop, a strategy which may be favored in periods of high density. We suggest that in order to understand the ecological and evolutionary significance of hilltopping, its populational aspects need to be investigated as well as individual behaviors, such as territoriality, which hilltopping butterflies may also exhibit."