The microbial composition, methanogenic activity and architecture of particulate and fixed biofilms within four anaerobic hybrid reactors, R1-R4, operating at psychrophilic temperatures were investigated. The reactors treated low-strength (1 g COD l(-1); R1) and high-strength (10 g COD l(-1); R2-R4) wastewaters from the food-processing sector (R1, R2 - whey; R3 - sucrose; R4 - volatile fatty acids). Successful start-up and long-term psychrophilic operation was observed for all four reactors, with COD removal efficiencies of 80-99% achieved at 12-20 degrees C at organic loading rates of 1.3-20 kg COD m(-3) d(-1). The formation and maintenance of a well-seftling granular sludge bed and an attached biofilm were shown to occur under psychrophilic conditions, an important consideration for the successful implementation of low temperature biofilm reactor technology. Culture-independent molecular techniques (terminal restriction fragment length polymorphism, clone library analysis and 16S rRNA gene sequencing) revealed that microbial population structure could be a key factor in reactor performance, with changes in the community structure of the three high-strength reactors preceding granular instability and a subsequent decline in COD removal efficiency. Biomonitoring of microbial population structure and dynamics within anaerobic reactors may, therefore, allow for the early recognition of potential operational problems.