The objectives of our study were to assess leaf-litter breakdown in 3 streams in 3 climates and to determine the contributions of associated microbial and invertebrate communities to the process. We incubated leaves of Alnus glutinosa in 1 stream in each of 3 climate zones: temperate (mountains of Central Portugal), Mediterranean (South Portugal), and tropical Cerrado (Minas Gerais, Brazil). Leaf-litter breakdown rates (/d) were faster in temperate (k = 0.023–0.017) than in tropical (k = 0.014) or Mediterranean (k = 0.014–0.009) streams. Leaf-litter breakdown rates (/degree day) also were higher in the temperate stream (k = 0.0018–0.0032) and similar between the other 2 streams (k = 0.008–0.0012). Colonization of leaves by aquatic hyphomycetes was faster in the temperate stream (maximum = 421 μg ergosterol/g of leaf by day 24) than in the tropical Cerrado or Mediterranean streams. However, peak ergosterol content was highest in the tropical Cerrado stream (573 μg/g on day 75). Ergosterol content was lowest in the Mediterranean stream (maximum = 341 μg/g on day 7). Total microbial biomass (as ATP) was higher in the tropical Cerrado stream (maximum = 531 nmoles/g on day 75) than in the Mediterranean (maximum = 108 nmoles/g on day 92) and temperate (maximum = 93 nmoles/g on day 7) streams. These results suggest either that not all microorganisms associated with leaves were involved in leaf-litter breakdown or that other less efficient microorganisms than fungi were involved in leaf-litter breakdown in the tropical stream. Leaves exposed to invertebrates (coarse-mesh bags) decomposed significantly faster than leaves protected from invertebrate feeding (fine-mesh bags) only in the temperate stream. This result suggests that invertebrates were important mediators of leaf-litter breakdown only in the temperate stream. A larger proportion of invertebrates recovered from decomposing leaves were shredders in the temperate stream (nearly 5%) than in the Mediterranean (1%) and tropical Cerrado (0%) streams. Leaf-litter processing rates increased with discharge and NO₃ concentration in the water. Our results suggest that the positive effect of temperature on breakdown rates of allochthonous organic matter in streams can be overridden by nutrient content in the water and the presence of invertebrate shredders.