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Variation in offspring size and number has been described for a wide range of organisms. Many theoretical models predict that in a given environment, the production of one single offspring size would yield the highest parental fitness. In most planktonic cladocerans, however, offspring size has been found to increase with size and age of the mother, and as individuals of variable size often co-occur within a population, offspring of variable sizes can be produced simultaneously. In this study, I investigated the relationship between age of the mother and size of her offspring to assess at what age of the mother the optimal offspring size was produced. Optimal offspring size was defined as that size of the offspring yielding the highest parental fitness, which translates to a definition of optimal offspring size as the one having the highest juvenile fitness per unit effort put in these juveniles. I observed that the youngest females produced offspring with the highest juvenile fitness per unit effort, and hence concluded that offspring produced by these females were of optimal size. Larger offspring produced by older females were estimated to yield only 70% of the potential fitness of optimally sized offspring.