The plant-produced indole alkaloid tryptamine is one of a large array of neuroactive substances that may affect insect behavior, development, and physiology. We tested the role of tryptamine on insect reproduction using the fruit fly, Drosophila melanogaster (Meigen), as a model system. Measurements were made of reproductive success, oviposition rate, and preadult survival of insects on artificial diets containing tryptamine, its precursor tryptophan, as well as glycine and serotonin (5-hydroxytryptamine). Drosophila reproduction was reduced to 15% of controls when adult insects mated and the young were allowed to develop on medium containing 75 mM tryptamine. Tryptamine-induced depression in reproductive success was due to decreased oviposition rate and preadult survival. Serotonin, but not tryptophan or glycine, also reduced oviposition rate. Preference tests indicated that tryptamine may act as an antiattractant or antifeedant in this species. The accumulation of the indole alkaloid tryptamine in plants may provide a mechanism for reducing insect reproduction, which is potentially useful in protecting crop plants.