<p>Bergmann’s rule, one of fundamental principles of macroecology, suggests that body size increases with latitude. To what extent do genetic changes and phenotypic plasticity interplay in shaping body size? We examined the presence of Bergmann’s rule in rotifers by comparing body size of seven littoral species from the Costa Rican, Polish, and Russian sites and included data for rotifers from artificially heated lakes in Poland to additionally test for the phenotypic plasticity effect. Four locations were selected: tropical waters in Costa Rica; temperate climates: Northwest Russia, the Great Masurian lakes, and artificially heated Konin lakes in Poland. We sampled rotifers in the warm period from the near-shore and vegetated parts of different water bodies at four sites and measured the length and width of lorica. We found that (i) rotifers from Costa Rica and the heated lakes were the smallest and (ii) each species showed or tended to show a decrease in body size with increasing temperature. This research adds to our knowledge of global body size patterns in zooplanktonic organisms and contributes to the discussion on the current interpretation of the Bergmann’s rule, which currently refers to temperature gradients rather than latitude.</p>