DIFFUSIBLE polypeptide pheromones (formerly referred to as mating-type factors, sex factors or gamones), which distinguish otherwise morphologically identical vegetative cell (mating) types from one another, are produced by some species of ciliates(1,2), Their most striking effect can be observed by exposing cells of one type to a pheromone secreted by another co-specific cell type(3). In the presence of this ’non-self’ signal, these cells interrupt their vegetative life to unite temporarily in mating pairs, Thus ciliate pheromones have traditionally been associated only with mating induction(2,4), However, the identification of autocrine pheromone receptors(5,6) suggests a broader role, which is supported by the hypothesis that ciliates evolved their mating-type mechanism for pursuing self-recognition(1). We now report studies, in the cosmopolitan marine sand-dwelling protozoan ciliate Euplotes raikovi, demonstrating that these molecules promote the vegetative reproduction (mitogenic proliferation or growth) of the same cells from which they originate, As, understandably, such autocrine pheromone activity is primary to that of targeting and inducing a foreign cell to mate (paracrine functions), this finding provides an example of how the original function of a molecule can be obscured during evolution by the acquisition of a new one.