Genetic control of decapitation-induced fission in planaria is mediated by stimulator and inhibitor molecules. Resolution of the two factors was accomplished by filtration chromatography of whole planaria extract. The stimulator (M.W. 2500 to 3500) was purified with ion exchange chromatography and increases the multiple fission rate. The inhibitor (M.W. 20,000 to 50,000) is non-dialyzable, heat labile and is inactivated unless kept frozen. Storage at 4°C results in a loss of inhibitory capacity with a concomitant stimulator increase, suggesting a protein-stimulator complex as the inhibitor. In light of a demonstrated animal intercommunication system, the inhibitor-stimulator complex is hypothesized as part of a natural population control mechanism. The stimulator (long-acting) is released into the culture medium and induces an all or none response among other members of the population which are physiologically receptive. Fission inhibition (short-lived) is elicited only upon intimate animal contact and is possibly in the animal slime.