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1. For two species of planktonic rotifers, Brachionus calyciflorus and Synchaeta pectinata, neonates from larger eggs were more resistantto starvation than neonates from smaller eggs, so egg size was positively correlated with offspring quality. 2. Theories of optimal offspring size in zooplankton predict either a decrease in offspring size with increasing food level, or maximum offspring size at intermediatefood levels. In experiments using acclimated, even-aged individuals,neither rotifer species changed its egg size as predicted by theory.B. calyciflorus egg size increased with increasing food concentration, while S. pectinata egg size did not change. 3. The lack of fit to theory may have occurred because of constraints on minimum or maximumegg size, constraints on total reproductive effort, or because offspring fitness is influenced by traits in addition to neonate starvation time. [ABSTRACT FROM AUTHOR]