There is concern about the ecological impacts that vast eucalyptus plantations in the Iberian Peninsula could have on streams, but evidence so far gathered is far from conclusive. In an attempt to check the effects of eucalyptus plantations while reducing inter-stream variability, we performed a manipulative experiment. In each of two small headwater streams, two 50-m long reaches were covered with 1-mm mesh net in order to exclude terrestrial inputs. We periodically added deciduous litter to the upstream reaches (deciduous treatments), and eucalyptus litter to the downstream ones (eucalyptus treatments), simulating the amount, composition and timing of inputs under both types of forest. Two reaches further upstream were used as controls to evaluate the effect produced by the exclusion mesh. We followed litter dynamics (inputs, storage ,outputs by transport and breakdown in bags) and macroinvertebrate density and biomass associated to bags and the benthos. Shredder density and biomass were affected negatively by the eucalyptus treatment, whereas total macroinvertebrates were not. Moreover, the comparison of the ratio bags/benthos for shredder density and biomass per organic matter revealed clear differences between treatments, showing a marked concentration of shredders in bags having good resource (alder) in the eucalyptus sites.