This paper uses energy relations to compare asexual and sexual reproduction and fission and egg production in freshwater triclads. Egg production is more efficient at converting food to reproductive energy but fission is more efficient at converting this to offspring Fission is therefore favored in ecological circumstances where food is always limiting and egg production in ecological circumstances where food is not alway s limiting In principle sex is the least efficient method of all the reproductive strategies yet partheno genesis and self-fertilization are rare. We cannot be sure, however to what extent the metabolic losses incurred by sperm production in hermaphrodites are made good by iesorption of the unused sperm denved from the mate. Furthermore, possible short-term and long-term balancing advantages of the sexual strategy can be identified in triclads.We also consider how, in those triclads which take the fission option, the body biomass should be divided between the head and tail in order to maximize the fitness of the system. If mortality is independent of size, a midway fission plane should be favored However strong size-dependent mortality occurs during spates in the streams in which fissiparous triclads are found, "pushing" the optimum fission plane tailwards.