1. Although ripe, fleshy fruits function primarily to attract seed dispersers, they must also be defended against diverse communities of seed predators and pathogens. For some plants, the concentration and diversity of secondary metabolites in fruits can exceed that of leaves and other plant parts, but little is known about the functional significance of the suites of compounds found in fruits. Fruit secondary metabolites may function in defense, or they may play a variety of other roles in seed development and dispersal. 2. In this study, we conducted a series of experiments to test the effects of amides, a highly diverse class of secondary metabolites found in Piper fruits, on a variety of antagonistic fruit pests, including an insect seed predator (Sibaria englemani, Pentatomidae) and three unidentified species of fungi isolated from ripe Piper reticulatum (Piperaceae) fruits. We tested the effects of amide-rich extracts from unripe and ripe fruits of Piper reticulatum and the effects of two pure compounds, piperine and piplartine, presented alone and in combination. 3. Amide-rich extracts from unripe and ripe fruits had no effect on insect feeding preferences, but strong negative effects on fungal growth rates. A comparison of the relative bioactivity of unripe and ripe fruit extracts, controlling for concentration, showed that the specific composition of compounds in unripe fruits provides a more effective defense than that of ripe fruits against two of the three fungal species tested. 4. Pure amides had variable effects on insect feeding preferences and strong negative effects on fungal growth rates. Tests of the bioactivity of two pure amides, presented alone and in combination, showed that the same two compounds can interact either synergistically or antagonistically in mixtures depending on the particular consumer involved. 5. Together, these results suggest that the secondary metabolites in fruits may be a key characteristic contributing to fruit defense and plant reproductive success. Specifically, our results emphasize: 1) the potential for slight changes in the composition of mixtures to alter the efficacy of defense, and 2) the potential for complex interactions among compounds in mixtures that can alter the bioactivity of secondary metabolites differentially among different consumers.