Climbing plants provide efficient pathways for ants to access patchy arboreal resources. However plant stems vary greatly in physical characteristics that are likely to influence ant locomotion. We collected measured and identified ants foraging on 671 stems of climbing plants at the La Selva Biological Station Costa Rica. We applied tuna baits to 70% of the observed stems to attract ants to a broad range of stem sizes. We used these data to examine relationships between relative stem roughness growth form (herbaceous or woody) stem diameter and the body length of foraging ants representing 58 species. The size of the largest ants found on stems generally increased with stem size up to 3.2mmdiameter whereas the size of the smallest ants present on stems did not vary with stem diameter. The largest ants in the forest (Paraponera clavata) used small stems (<2.7 mm diameter) only when attracted by baits. Average (± SE) ant body length was larger on woody (5.2 ± 0.32 mm) vs. herbaceous (3.3 ± 0.53 mm) stems but did not differ between rough and smooth stems within these categories. Ant body-size distribution tended toward unimodality on smooth stems. We conclude that small stem diameter acts as a habitat filter based on ant body size but only for the largest ants in the forest. The filter effect is reduced when ants are attracted to an artificially high quality resource.