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Acanthocyclops robustus is a freshwater cyclopoid copepod that posses a complex life cycle and presents large differences between males and females, as well as between different growth stages. It has omnivorous and cannibal food habits, it can feed on many kinds of organisms like microalgae, rotifers, cladocerans, and even their own nauplii. In this study, the selective predation by the adult of this copepod on the two rotifers Keratella cochlearis and Brachionus calyciflorus was studied in the laboratory, using, monoculture or mixed diets, at different prey concentrations. The results show that Brachionus is positively selected by the copepod in mixed diets. Moreover, the cannibalism exerted by the adult copepod female on their own nauplii was analyzed. Two starting hypotheses on the cannibal behaviour of A. robustus were proposed: 1) the longer the time period since egg hatching, the lower the predation rate on the nauplii will be; and 2) the higher the food concentration (Keratella individuals) for the adult copepod in the culture, the lower the predation rates on the nauplii. In the experiments performed using different food concentrations and different exposure time, statistically significant differences were found on the nauplii predation rate due to the time period, but not due to food concentration in the culture. The adult stage of A. robustus has a type II functional response not only in their feeding on rotifers but also in their cannibalism; it predates nauplii even when big and mobile, but it does not select them with respect to the rest of the food present.