The impacts of simulated climate change (warming and fertilization treatments) on diazotroph community structure and activity were investigated at Alexandra Fiord, Ellesmere Island, Canada. Open Top Chambers were randomly placed in a dwarf-shrub, cushion-plant dominated mesic tundra site in 1995, these raised growing season temperatures by 1-3°C. In 2000 and 2001 20N:20P₂O₅:20K₂O fertilizer was applied at a rate of 5 g m^-2 year^-1. Estimates of nitrogen fixation rates were made in the field by Acetylene Reduction Assays (ARA). Higher rates of N fixation were observed 19-35 days post-fertilization but were otherwise unaffected by treatments. Moss cover was significantly positively associated with ARA rate indicating the importance of cyanobacteria at this site. NifH gene variants were amplified from bulk soil DNA and analyzed by Terminal Restriction Fragment Length Polymorphism (TRFLP) analysis. Nonmetric Multidimensional Scaling was used to ordinate treatment plots in nifH genotype space. NifH gene communities were more strongly structured by the warming treatment late in the growing season, suggesting that an annual succession in diazotroph community composition occurs. T-RFs common to the actinobacteria, the Alpha-proteobacteria and the cyanobacteria were all significantly associated with warming treatments. Climate warming will likely increase biological nitrogen fixation at this site as long as vascular plant growth is nutrient constrained.