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The effect of high pH on the reproduction of two Daphnia galeata clones was experimentally investigated in the laboratory. We observed that the mortality of juveniles and adults did not increase with increasing pH in the range pH 9.0–10.5, which agrees with what is generally reported in the literature for cladoceran zooplankton and suggests that the threshold value for mortality is between pH 10.5 and 11.5. However, both egg mortality and the proportion of stillborn neonates increased at pH 10.0 and above, and the two clones differed in their sensitivities to pH. Consequently, pH already affects population growth rate markedly from pH 10.0 onward. Because pH values ≥ 10.0 are common during spring and summer in many eutrophic and hypertrophic lakes due to intense photosynthetic activity, we expect that high pH has a much larger effect on the population structure of Daphnia and the community composition of microcrustacean zooplankton in such water bodies than was assumed previously.