Carbon and nitrogen stable isotope analysis was used to study the reproductive contributions of anadromous and freshwater resident female brown trout (Salmo trutta L.) in La Roche Brook (Normandy, France), where both forms live in sympatry. Sixty-nine emerging fry were sampled by electrofishing in three different locations in the lower, middle and upper reaches of the stream, in March–April 2002. A collection of samples with known origin (brown trout eggs and fin tissues and benthic invertebrates) allowed classification of each emerging fry as the progeny of freshwater resident (δ13C = –26.0‰ ± 0.8 s.d.; δ15N = 12.5‰ ± 1.4 s.d.) or anadromous (δ13C = –18.8‰ ± 0.6 s.d., δ15N = 14.8‰ ± 0.9 s.d.) female brown trout. It was then possible to estimate the relative contribution (33%) of anadromous females to the population of fry that emerged from redds in 2002 and to evaluate the extent to which freshwater and anadromous spawning grounds overlapped in the stream.