The replacement of native vegetation by exotic species has the potential to impact detritus based stream ecosystems. To test the possible consequences of vegetation alteration on stream ecosystem functioning we com-pared the processing rate and invertebrate colonisation of leaves from fi ve different tree species, two natives (Alnus glutinosa and Quercus pyrenaica) and three exotics (Populus × canadensis, Platanus hispanica and Pinus radiata) in three unaltered forested streams in the Atlantic region of northern Spain. Breakdown rates, nitrogen concentra-tion and C:N ratio of incubated leaves, and invertebrate communities in the benthos and bags were analysed. Break-down rates differed among leaf species and were positively related to initial nitrogen concentration and negatively to the C:N ratio. After the incubation period (6 –7 weeks), the differences in nutritional quality among leaf species and the relationship between breakdown rates and litter quality persisted. The density and biomass of macroinverte-brates and shredders associated with litter bags correlated with leaf-nitrogen concentration and explained the differ-ences in breakdown rates. The results indicate that the replacement of native deciduous forest by plant commercial species with leaf-litter of low quality such as plane and pine can affect in-stream organic matter processing and the subsequent transference of energy throughout the food web.