The influence of inbreeding was studied in the freshwater planarianDugesia polychroawhich, in addition to reproduce by copulation, is able to form self‐pairs following experimentally induced regeneration. A comparative analysis of the degree of fertility of cocoons laid by cross‐pairs and self‐pairs, showed that cocoons from the latter contain remarkably fewer juveniles which, in addition, are less viable. These results were considered to reflect a Darwinian effect of inbreeding depression.