Literature on local genetic differentiation in freshwater zooplankton populations is reviewed The island-like nature of limnetic habitats creates opportunities for local genetic differentiation and adaptation to develop. There is a wealth of data available on genetic differentiation among populations of zooplankton with respect to allozyme markers. Data from well-designed studies on ecologically relevant, quantitative traits are less abundant and indicate a different pattern from that obtained using electrophoretic markers. It is argued that whereas the analysis of (quasi) neutral markers emphasizes the importance of long-lasting founder effects and genetic drift, the pattern of local genetic differentiation of ecologically relevant traits may often reflect local adaptation. In reviewing the data, the importance of temporal and spatial habitat selection in maintaining genetic polymorphism for ecologically relevant traits is emphasized, without denying the importance of stochasticity. Most available data are on Daphnia, but studies on other organisms in general confirm the patterns observed in this genus. A hypothetical scheme of the processes leading to local genetic differentiation and adaptation in zooplankton is discussed, with an indication of the data necessary to fill certain gaps in our knowledge. Attention is drawn to the frequent opportunities for local adaptation in cyclically parthenogenetic organisms (e.g., Daphnia, monogonont rotifers) and the processes leading to local adaptation in cyclically parthenogenetic, obligately parthenogenetic and obligately sexual species are compared.