Four dominant nematode species and various copepods from an intertidal mudflat in Marennes-Oléron Bay were analyzed for δ13C and δ15N ratios together with their potential food source, the microphytobenthos. It was shown that microphytobenthic C is passed to nematodes either by grazing or predation on herbivorous meiofauna by other nematodes. Diatoms of different sizes exhibited different 13C content, which can be reflected in herbivorous meiofauna, such as the nematodes Daptonema oxycerca and Metachromadora remanei and the bulk of copepods, indicating they can select different sizes of diatoms. Results confirm the need to analyze meiofauna feeding habits at the species or genera level since the δ15N and δ13C signatures differed substantially among meiofaunal species