Trophic interactions within the plankton of the lowland river Meuse (Belgium) were measured in spring and summer 2001. Consumption of bacteria by protozoa was measured by monitoring the disappearance of 3H-thymidine labelled bacteria. Metazooplankton bacterivory was assessed using 0.5 µm fluorescent microparticles (FMPs) and predation of metazooplankton on ciliates was measured by using natural ciliate assemblages labelled with FMPs as tracer food. Grazing of metazooplankton on flagellates was determined through in situ incubations with manipulated metazooplankton densities. Protozooplankton bacterivory varied between 6.08 and 53.90 mg C m-3 d-1. i.e. from 0,12 to 0,86 gC-1 bacteria gC-1 protozoa d-1. Metazooplankton, essentially rotifers, grazing on bacteria was negligible compared to grazing by protozoa (∼1000 times lower). Predation of rotifers on heterotrophic flagellates (HF) was generally low (on average 1.77 mg C m-3 d-1 i.e. 0,084 gC-1 flagellates gC-1 rotifers d-1), the higher contribution of HF in the diet of rotifers being observed when Keratella cochlearis was the dominant metazooplankter. Predation of rotifers on ciliates was low in spring samples (0.56 mg C m-3 d-1 i.e. 0,014 gC-1 ciliates gC-1 rotifers d-1) in contrast to measurements performed in July (8.72 mgC m-3 d-1 i.e. 0,242 gC-1 ciliates gC-1 rotifers d-1). The proportion of protozoa in the diet of rotifers was low compared to that of phytoplankton (< 30% of total carbon ingestion) except when phytoplankton biomass decreased below the incipient limiting level (ILL) of the main metazooplantonic species. In such conditions, protozoa (mainly ciliates) constituted ca. 50% of total rotifer diet. These results give evidence that microbial organisms play a significant role within the planktonic foodweb of a eutrophic lowland river, ciliates providing an alternative food for metazooplankton when phytoplankton becomes scarce.