During a re-examination of macrozoobenthic samples from a long-term monitoring programme in the Dutch Oosterschelde and Westerschelde, the abundant presence of peritrich ciliates on specimens ofBathyporeiaLindström, 1855, was discovered. Out of the more than 3500Bathyporeiaspecimens investigated, 44% contained ciliates. AlthoughBathyporeia sarsiWatkin, 1938 was significantly more often infested thanBathyporeia pilosaLindström, 1855, these differences in infestation rates were largely due to differences between water bodies with higher infestation rates in the polyhaline than in the mesohaline reach. Observation of additionally collected living specimens and freshly preserved material showed that at least two, and likely three, species of ciliates are present of which two might be undescribed so far. One of the observed species matchesZoothamnium nanumKahl, 1933. A second species belongs to the genusEpistylisEhrenberg, 1830, but does not seem to match a so far known species. This also accounts for a possible third species belonging to the genusZoothamniumBory de St. Vincent, 1826, deviating fromZ. nanumamongst others in the habitus of the stalk. The front part ofBathyporeiaspp. and the antennae in particular, significantly more often harboured ciliates than the remainder of the body, where additional ciliates were only found on the ventral side. This shows that the peritrich ciliates benefit from the water currents induced by the basibiont, providing food items, but might also indicate thatBathyporeiaspp. benefits from the presence of the epibionts as they are most prevalent on the body parts that are easiest to clean. Analyses of densities and distributions of epibiont and basibiont species gave first indications of the ecological niche of the peritrich ciliate communities.