Brown trout Salmo trutta and Atlantic salmon Salmo salar can interbreed and produce viable hybrid offspring. Literature indicates that maternal species can either be brown trout (North America) or Atlantic salmon (South- ern Europe and Ireland), and bidirectional hybridization has also been reported (England, Northern Europe and subantarctic French Territory). In coastal rivers where both species are sympatric, brown trout populations often split into two morphs, stream residents and anadromous (sea-run) migrants. No nuclear DNA marker is available for identification of the two morphs, but Stable Isotope Analysis (SIA) has recently proven useful for determina- tion of maternal anadromous life histories among recently emerged fry. Here we combine genetic analysis and SIA to investigate whether brown trout females of the two morphs may interbreed with Atlantic salmon. In spring 2003, 88 recently emerged fry were collected in La Roche brook (Oir River, Normandy, France), a second order stream that provides spawning grounds for brown trout (freshwater resident and anadromous) and A tlantic salmon. First, microsatellite DNA (6 loci) analysis was carried out to determine the frequency of hybr ids in the sample. Two hybrids were found (frequency = 2.3%). Second, the maternal migratory origin anadromous or freshwater resident) of fry was inferred from SIA (d13C and d15N). One hybrid arose from a freshwater resident female brown trout x Atlantic salmon male cross. The second hybrid has a maternal anadromous origin, but SIA failed in the attempt to identify maternal species (brown trout or Atlantic salmon). Mitochondrial DNA analysis is being performed to clarify this issue. It is suggested that a combination of appropriate tools like SIA and genetic analyses can improve our understanding of complex interactions between anadromous and non-anadromous fish populations.