Macrobiofouling is an important phenomenon in the aquatic environment, resulting in economic losses and environmental changes, including negative impact on hard‐shelled animals. A freshwater invasive byssate bivalve, the zebra mussel Dreissena polymorpha (Dreissenidae), strongly affects bivalves from the family Unionidae by fouling their shells. We tested potential mechanisms explaining variable fouling of different unionids (four native species: Anodonta anatina, A. cygnea, Unio pictorum, U. tumidus and the non‐native Sinanodonta woodiana) by the zebra mussel. We conducted a field survey (unionids collected at a sandy and muddy site in a dam reservoir), field experiment (unionids exposed without sediments in suspended baskets to dreissenid settlement) and laboratory experiment (a multiple choice test with and without the possibility of unionid burrowing into sandy sediments). In the survey, zebra mussel density was highest on A. anatina and S. woodiana, intermediate on U. pictorum and lowest on A. cygnea and U. tumidus. In the field experiment, A. anatina and S. woodiana were more fouled than Unio spp. In the laboratory, zebra mussels less often attached to U. pictorum and, when unionids could burrow themselves, also to S. woodiana. However, no unionid species was positively selected in the presence of stone substrata. The percentage of zebra mussels attached directly to unionids (compared to those attached to conspecifics) in the field survey was negatively related to the overall zebra mussel biomass. Zebra mussel fouling negatively affected the biomass of an allopatric S. woodiana on muddy bottom. Dreissenids overgrow different unionid species to variable extent, not only due to the differences in their exposed surface area, but, as our experimental results show, also because of their active substratum selection. Moreover, mussels seem to prefer unionid surface over conspecific shells, the latter being fouled only when overall dreissenid fouling is heavy. As unionids often constitute the main source of hard substratum for zebra mussels in waterbodies, the species composition of a unionid assemblage may affect dreissenid success by offering them variable substratum quality.