Bacterial symbionts produce small molecules that exert profound influences on animal health,morphogenesis,and evolution through poorly understood mechanisms.This lecture begins with recent discoveries about how a small bacterially produced molecule,RIF-1,causes a model organism to develop multicellular colonies from single-celled precursors.RIF-1 has both extraordinary potency(fM) and a broad dynamic range(109).This model for bacterially induced development has implications for the ways in which bacterial symbionts affect animal development,including humans,that suggest avenues through which new chemicstry and biology can be discovered.