This version thus not correspond to the final version of the journal. To assess the final version please go to: http://www.sciencedirect.com/science/article/pii/S1754504812000876 ; Climate warming and biodiversity loss are two major factors threatening freshwaters. Aquatic hyphomycetes are fungi that play a key role in organic matter turnover in streams. To assess the impacts of temperature increase and aquatic hyphomycete diversity on plant-litter decomposition, we manipulated fungal assemblage composition at two levels of diversity (four and eight species) under ambient temperature of 16 º C and two regimes of temperature increase differing in 8 º C: abrupt versus gradual increase from 16 to 24 º C. The effects were evaluated on leaf-litter decomposition, fungal biomass and reproduction. Results showed faster leaf decomposition under increased temperature, but no differences were found between an abrupt and a gradual increase in temperature. Assemblage composition was the major factor controlling fungal biomass and reproduction, while fungal diversity was only critical to maintain reproduction.