{Canyon-shaped reservoirs are often characterised by longitudinal gradients of environmental factors (including trophic level, phytoplankton and zooplankton biomass and abundance of planktivorous fish) affecting the taxonomic composition of the pelagic community. We tested the hypothesis that the spatial distribution of different species and interspecific hybrids of the Daphnia longispina species complex is non-random under such conditions. During the summer stratification, we sampled crustacean zooplankton from 11 reservoirs, covering both longitudinal (upstream, middle, dam) and vertical (epi-, meta- and hypolimnion) environmental gradients. Allozyme electrophoresis was used to discriminate among different Daphnia taxa. All three frequently hybridizing European species of the complex (galeata, cucullata