Previous work in an alpine dry meadow in the Front Range of the Rocky Mountains has shown that microbial biomass is high during winter and declines rapidly as snow melts in the spring, and that this decline is associated with changes in temperature regime and substrate availability. In this study we tested the hypothesis that the summer and winter microbial communities differ in function and composition. Shifts in species composition between pre- and post-snowmelt communities were detected using reciprocal hybridization of community DNA; DNA extracted from soils sampled at different times was significantly less homologous relative to spatial replicates sampled at the same time. Fungal/bacterial ratios, as measured by direct microscopic counts and by substrate-induced respiration experiments with specific inhibitors, were higher in winter soils. Specific activity of cellulase (absolute cellulase activity per unit microbial biomass C) was higher in the winter soils than in summer soils, while specific amylase activity was not different between winter and summer. Based on most-probable number measurements, the use of the phenolic compound vanillic acid was highest in the winter, while the use of the amino acid glycine was lowest in the winter. Winter and summer soil respiration responded differently to temperature; at 0 degrees C, winter soils respired at a higher proportion of the 22 degrees C rate than did summer soils.