Abstract Samples of water, sediment and bacterial mat from hot springs in Grændalur and Hveragerdi areas in southwestern Iceland were screened at 70°C and 80°C for thermophilic denitrifying bacteria by culturing in anaerobic media containing nitrate or N 2O as the terminal oxidant. The s springs ranged in temperature from 65–100°C and included both neutral (pH 7–8.5) and acidic (pH 2.5–4) types. Nitrate reducing bacteria (nitrate → nitrite) and denitrifiers (nitrate → N 2) were found that grew at 70°C but not at 80°C in nutrient media at pH 8. Samples from neutral springs that were cultured at pH 8 failed to yield a chemolithotrophic, sulfur-oxidizing and nitrate-reducing bacterium, and samples from acidic springs that were cultured at pH 3.5 seemed entirely to lack dissimilatory, nitrate-utilizing bacteria. No sample yielded an organism capable of growth solely by N 2O respiration. The denitrifiers appeared to be Bacillus. Two such Bacillus strains were examined in pure culture and found to exhibit the unusual denitrification phenotype described previously for the mesophile, Pseudomonas aeruginosa, and one other strain of thermophilic Bacillus. The phenotype is characterized by the ability to grow by reduction of nitrate to N 2 with N 2O as an intermediate but a virtual inability to reduce N 2O when N 2O was the sole oxidant.