Thermophilic acetate-utilizing bacteria were isolated by using in situ enrichment methods. The bacteria were enriched in a hot spring by feeding 3 mol/l ammonium acetate or 3 mol/l ammonium acetate with 0.5 mol/l sodium sulfite semi-continuously into the hot spring for several weeks. The bacteria were then isolated in the laboratory by conventional enrichment and plating techniques. Bacteria were also isolated from a wood pulping mill. The isolates could grow at 65 °C on 12.8–18 g/l of ammonium acetate. Some strains grew even with 2.5 g/l of sodium sulfite added, as well as in media made out of evaporator condensate produced at a pulp mill. All isolates which grew in the highest concentrations of ammonium acetate were found to be Gram-positive, sporeforming, alkalophilic bacilli.