AbstractPopulations of Erpobdella octoculata and E. vilnensis were studied monthly over two years (May 1999–April 2001) at four sampling points on the river Rokytná (Czech Republic). The water depth and velocity were measured and the Froude number for each sample was calculated to follow flow conditions above the bottom. Temporal differences in biomass of the two species were studied by comparing their wet weights and evaluated by the time series analysis; differences in their diets were evaluated by gut content analyses. Significantly different hydrodynamic preferences for juvenile and adult stages of E. octoculata and E. vilnensis were recorded. In E. vilnensis only, an annual cycle was recorded with a one‐month shift of mean weights compared to E. octoculata. This shift, together with the correlation of the leeches wet weights with their prey sizes and their distinct habitat preferences are suggested as mechanisms that explain their coexistence. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)