The present study explores the use of stable carbon isotopes to trace organic matter sources of intertidal nematodes in the Schelde estuary (SW Netherlands). Stable carbon isotope signatures of nematodes from a saltmarsh and 4 tidal flat stations were determined in spring and winter situations, and compared to isotope ratios of organic matter sources within the estuary. Nematodes collected from fine sandy tidal flat sediments in late spring and during mild and sunny late winter weather had delta 13C values consistent with microphytobenthos as their prime carbon source. Nematodes from a silty station and individuals sampled under cold and dark winter conditions had delta 13C values intermediate between those of microalgae and particulate organic matter. The isotopic signatures of nematodes from the saltmarsh station were intermediate between those of microalgae, Spartina anglica -litter and particulate organic matter. An in situ H13 CO3- spike experiment at 2 tidal flat stations corroborated the importance of microphytobenthos as a carbon source for nematodes, yet at the same time contradicted the hypothesis that direct grazing would be the main pathway of microalgal carbon to nematode consumers. A laboratory experiment adding 13C-labeled algal detritus to sediment microcosms demonstrated the nematodes' ability to rapidly utilize settling organic matter too. Incorporation of carbon from phytodetritus by subsurface nematodes in both enrichment experiments was high, but, compared to carbon utilization by surface individuals, showed time-lags largely consistent with sediment mixing rates. A combination of observed natural isotope signatures and experimental results suggests that tidal flat nematodes preferentially utilize labile organic carbon derived from microphytobenthos or settling phytoplankton; organic matter from terrestrial or riverine origin does not contribute significantly to the diet of nematodes at our study sites.