1. We examined changes in leaf structure and photosynthetic characteristics in two congeneric shrub species growing across natural gap transects in a tropical rain forest. 2. Piper sancti-felicis, a shrub characteristic of disturbed clearings and large gaps, exhibits a high capacity to alter photosynthetic capacity in response to spatial variation in light availability compared to the shade-tolerant species Piper arieianum. 3. Rooted cuttings were placed in pots along transects in each of two recent, naturally formed canopy gaps in old-growth forest. After 13 months of growth, a subsample of 14 plants per species (seven pairs per gap) was chosen for detailed analyses of leaf light environment and photosynthetic light responses. For 10 of these pairs, leaf tissue was sampled for subsequent anatomical characteristics. 4. Photosynthetic capacity per unit leaf area of Piper sancti-felicis showed a strong dependence on all measures of light availability, whereas Piper arieianum showed a weak or insignificant light acclimation response. Photosynthetic capacity and dark respiration per unit leaf mass increased significantly with light availability only for Piper sancti-felicis. Both species exhibited significant light-dependent variation in leaf mass per area and thickness of mesophyll layers, although only Piper arieianum showed light-related variation in total leaf thickness. Photosynthetic capacity of Piper arieianum was poorly correlated with leaf anatomical features, whereas leaves of Piper sancti-felicis showed significant, positive correlations between photosynthetic capacity and mesophyll tissue thickness. The limited potential for light acclimation in the shade species Piper arieianum reflects an inability to adjust metabolic processes at the cellular level, despite extensive anatomical adjustments. Light acclimation in the high-light species Piper sancti-felicis, in contrast, is effected by biochemical adjustments at the cellular level as well as by changes in leaf anatomy.