Stable carbon and nitrogen isotope ratios were used to define the trophic base of the infaunal suspension-feeding bivalve Cerastoderma edule (L.) living on an intertidal muddy sandflat in Marennes-Oléron Bay, France. Suspended particulate organic matter (POM) collected from adjacent marine channels had a mean δ13C value of -22.2 ± 1.1‰ (n = 19). Benthic primary producers on Ronce-les-Bains tidal flats had δ13C values distinct from that of the POM: microphytobenthos (-16.0 ± 0.6‰), the macroalgae Enteromorpha compressa (-17.6 ± 2.8‰), Fucus serratus (-17.4 ± 2.6‰), Fucus vesiculosus (-18.5 ± 1.6‰), Phorphyra umbilicalis (-19.7 ± 0.5‰), Ulva rigida (-14.7 ± 2.0‰) and the seagrass Zostera noltii (-11.1 ± 1.0‰). Mean δ15N values of all macroalgae species ranged from 7.6 ± 1.3 to 8.9 ± 1.0‰, while those for microphytobenthos, POM and seagrass were 5.3 ± 0.8, 5.0 ± 0.9 and 6.5 ± 1.3‰, respectively. The mean δ13C value of spat (-15.3 ± 0.8‰) and juvenile cockles (-15.7 ± 0.7‰) varied within a smaller range than those of 1 to 4 yr old adults (-18.2 ± 1.2‰). The δ15N values of 0-group cockles (spat and juveniles) and adult cockles are similar (8.0 ± 0.9‰ for 0-group and 8.4 ± 1.1‰ for adults). The results suggest, based on the average trophic enrichment found in the literature for C and N, and the relative abundance of each food source, that there are 2 major sources of organic matter assimilated by cockles of the studied flats: microphytobenthos and POM. Seasonal variations in δ13C values reflect a higher dependence of adult cockles on POM variability while spat and juveniles are more closely linked to microphytobenthos. The isotopic shifts indicate that the relative importance of the 2 major food sources depends on the age of the cockles and the season.