In this study we aimed to investigate the utility of the barn owl diet as a tool for determining the spatial distribution and abundance of the yellow-necked mouse, a forest-dwelling small mammal species, in the Montseny mountains and surrounding plains (Barcelona, NE Spain). After validating the molar shape criterion for species discrimination (t9 of second upper molar), we classified a sample of 2,684 Apodemus spp. identified from 8,831 small mammals of 17 species from barn owl pellets collected in 18 roosting places. Almost all individuals were determined as A. sylvaticus (91.8%) and the remainder were considered as A. flavicollis (8.2%). Frequencies of occurrence of A. flavicollis were related to 11 environmental descriptors by means of Redundancy Analysis and partial constrained ordination. Frequencies of occurrence changed along a strong environmental gradient (increased towards rainy localities covered by deciduous forests), and this pattern coincided with that observed for other forest predator (Genetta genetta). The use of non-invasive sampling methods, like the barn owl diet analysis, can be more efficient in detecting the presence of the yellow-necked mouse than other conventional sampling methods, since accurate species determination of the binomial A. sylvaticus/A. flavicollis is difficult after live-trapping in NE Spain.