The leaves of tropical forest trees are most likely to suffer herbivore damage during the period of expansion. Herbivore selection on young leaves has given rise to a variety of leaf developmental strategies and age-specific chemical defense modes. We are studying correlations between leaf developmental types and chemical defenses in the Neotropical genus Inga. We have characterized defense metabolites in Inga goldmanii and Inga umbellifera, two species that co-occur in the lowland moist forest of Panama. These congeners have markedly different young-leaf developmental phenotypes but suffer approximately equal rates of herbivory. Bioassays of whole and fractionated leaf extracts using larvae of Heliothis virescens show that I. goldmanii chemical defenses are nearly three times more inhibitory than those of I. umbellifera. In both species, most of the inhibitory activity resides in complex mixtures of monomeric and polymeric flavan-3-ols. This group comprises >30% of young leaf dry weight in both I. goldmanii and I. umbellifera. The species' phenolic chemistry differs markedly, however, both in the structure of the monomeric units and in the distribution of polymer sizes. The differences in chemical structure have pronounced effects on their bioactivities, with I. goldmanii flavans being twice as inhibitory to H. virescens larvae as I. umbellifera flavans, and more than three times more efficient at protein binding. Given the extraordinarily high polyphenol concentrations that are found in the young leaves of these species, protein precipitation could be an important mechanism of growth inhibition. Nevertheless, our data show that another mode of phenolic action, possibly oxidative stress, occurs simultaneously.