We used stable isotope analysis in an attempt to detect marine subsidies from anadromous fish to freshwater benthos in four river systems draining to the Atlantic Ocean. Benthic invertebrates in the West River, Nova Scotia, Canada, had elevated d13C, d15N, and d34S values in a downstream reach that suggested consumption of marine-derived organic matter from spawning blueback herring Alosa aestivalis. In Doctor's Brook, Nova Scotia, the arrival of rainbow smelt Osmerus mordax to spawn led to rapid increases in the d13C and d15N of a predatory stonefly (Perlidae), but lower trophic levels (mayflies and biofilm) showed inconsistent responses. Sculpin Cottus sp. showed no evidence of predation on Atlantic salmon Salmo salar eggs in Catamaran Brook, New Brunswick, Canada or the Scorff River, Brittany, France. These analyses suggest that marine organic matter subsidies, in the form of direct consumption of eggs and/or carcasses, are important in streams with concentrated spawning activity such as by alosid and osmerid species, whereas carbon and nitrogen contributions from more sparse spawning species such as by Atlantic salmon may be minimal.