Territorial systems of Neotropical birds remain poorly understood, especially considering the effect of interspecific interactions. Studies on territorial behavior provide information about demography and life history, and are useful for the refinement of conservation strategies. We assessed territories of six species of antbirds (Thamnophilidae) from August 2010 to May 2011 at Pedra Azul State Park, a protected area in southeastern Brazil, in a 30-ha sampling grid. Birds were captured, marked, and followed through the study area. Territory sizes of Plain Antvireo (Dysithamnus mentalis), Ferruginous Antbird (Drymophila ferruginea), Ochre-rumped Antbird (Drymophila ochropyga), White-bibbed Antbird (Myrmeciza loricata), White-shouldered Fire-eye (Pyriglena leucoptera), and Variable Antshrike (Thamnophilus caerulescens) were estimated by the convex polygon method. We found 46 territories of the six species. We compared and evaluated the effect of body size on territories among the species. Males and females were active in territory defense by emitting songs and calls. The mean territory sizes defended by the six species varied from 0.72–1.18 ha. We found empty spaces between territories and a high overlap rate among territories of different species, but no overlap within species. Thamnophilus caerulescens had the highest density and D. mentalis and D. ferruginea had lower densities. Body mass and distance to the forest edge had no effect on territory size. The territory assessment provided a precise density estimation of individuals in the study area.