Coastal and estuarine nurseries are essential habitats for juvenile flatfish. These small but productive areas provide food supply and sustain adult fish populations. The Mont-Saint-Michel Bay (MSMB) supports an important flatfish nursery ground but differs from many other nursery habitats due to limited freshwater inputs. The objectives of the present study in the MSMB were to (1) use gut content analysis to identify prey of the 2 most abundant flatfish species (common sole Solea solea and plaice Pleuronectes platessa) for different juvenile age-classes (0-group and 1-group for sole, 0-group for plaice), (2) use C and N stable isotope analysis to model the production sources sustaining juvenile flatfish production, and (3) compare these results with previous knowledge of estuarine nursery grounds located in western Europe. Items found in gut contents differed between species and size classes, with juvenile plaice having a larger prey spectrum. Despite accounting for sensitivity to large uncertainties in source signatures and trophic enrichment factors, stable isotope mixing models led to robust outputs. In contrast to previous studies in estuarine nurseries, we found that microphytobenthos was the major carbon source contribution to juvenile flatfish. The contribution of freshwater particulate organic matter was nonetheless significant, in spite of the very limited river inputs to MSMB.