Decline in pH, elevated aluminium (Al) concentrations, and base cations depletion often covary in acidified headwater streams. These parameters are considered as the main factors reducing microbial activities involved in detritus processing, but their individual and interactive effects are still unclear. In addition to its direct toxicity, Al can also reduce the bioavailability of phosphorus (P) in ecosystems through the formation of stable chemical complexes. A three week microcosm experiment was carried out in acid conditions to assess the interactive effects of Al (three levels: 0, 200, and 1000 mu g L-1) and P (25, 100, and 1000 mu g L-1) on alder leaf litter processing by an aquatic hyphomycete consortium. Our results showed that Al alone reduced fungal growth and altered fungal decomposer activities. High P levels, probably through an alleviation of Al-induced P limitation and a reduction of Al toxic forms, suppressed the negative effects of Al on detritus decomposition.