This study analyzed the influence of vegetation structure variation along a natural vegetation mosaic formed by Araucaria forest and Campos grassland in the southern Brazilian highlands, on the species richness and composition of epigeic ants. The study site consisted of two different grassland–forest ecotones, where 76 pitfall traps were installed. We estimated the area covered by canopy vegetation by hemispherical photographs, and the structure of the understory vegetation by counting the number of vegetation touches, using a graduated stick. We collected 31 species or morphospecies of epigeic ants belonging to 17 genera and 6 subfamilies. Cluster analysis defined three habitat groups (grassland, edge, and forest) with different ant species composition as revealed by ordination analysis. The highest richness was observed at the forest edge, and decreased towards the grassland and the forest interior. Variation in the richness and composition of epigeic ant species was significantly explained by the factor of distance from the forest. The relationship between species richness and understory density was negative. On the other hand, species composition of epigeic ant assemblages was significantly explained by canopy cover. This finding indicates that the ecological responses of ant assemblages resulted predominantly from edge effects mediated by changes in vegetation structure.