A model which defines fitness in terms of rate of clone increase, and which assumes constant mortality, predicts that more smaller offspring should be produced in conditions which are good for individual growth. Evidence from the Protozoa, freshwater flatworms and anthozoan cnidarians support this prediction. Fitness is maximized by budding (or paratomous fission) if adult survivorship or growth is unaffected by offspring attachment to parent but time to first breeding is thereby reduced. Whether or not the adult is affected by offspring attachment is likely to depend on how mobile the adult is, so we assess the occurrence of offspring attachment in relation to adult mobility. It seems that budding is associated with a sessile life-style in the ciliated Protozoa, and usually in the cnidarians. Paratomy is associated with an interstitial existence in the mobile acoel and rhabdocoel turbellarians and the aeolosomatid and naidid annelids. Budding is rare in the solitary Anthozoa and the holothuroidean echinoderms, both of which are sessile. However in these animals survival of the adult is dependent on whole body contractions and would be impaired by the existence of buds. We predict what proportion of adult resources should be devoted to reproduction, and we consider the complication of encapsulation.