Root-associated bacteria capable of reducing acetylene to ethylene (biological nitrogen fixation) were isolated from various native plants grown in the Canadian High Arctic. All the strains belonged to the genus Pseudomonas but varied in several physiological characteristics. The rates of acetylene reduction at 14 or 20 degrees C were higher than at 25 or 9 degrees C. Six strains reduced acetylene at 4 degrees C. All the strains exhibited chemotaxis to l-asparagine in semisolid agar at 4 to 25 degrees C. Eleven strains colonized roots of canola (Brassica campestris cv. Tobin) in field soil at population densities of log 4.3 to log 5.1 CFU/g of fresh root at 14 degrees C and log 4.0 to log 5.2 CFU/g of fresh root at 25 degrees C. Some of these nitrogen-fixing pseudomonad strains demonstrated a competitive advantage for root colonization over other rhizosphere bacteria at low temperatures. The combined capabilities of nitrogen fixation and root colonization by diazotrophic pseudomonads may be useful for the development of a biofertilizer inoculant for temperate and cold regions.