Life-cycle theory predicts that a trade-off exists between somatic maintenance and reproduction. Short-lived semelparous organisms, with a high investment in reproduction, should have lower levels of somatic cell turnover leading to accelerated senescence and reduced post-reproductive survival, than long-lived iteroparous organisms. This model has been tested by intraspecific comparison of mitotic rates and rates of regeneration, taken as indexes of somatic cell population dynamics, between two different populations of a reportedly semelparous species of freshwater triclad,Dendrocoelum lacteum, and a population of the iteroparous speciesDugesia(S)mediterranea.