A Thermus strain, producing an extracellular protease, was isolated in a hot spring in Iceland. The main growth characteristics of this isolate were studied with different cultivation vessels and different cultivation techniques. A clear and striking dependence of the growth behavior on the cultivation technique was apparent. Higher maximum yield of biomass, higher productivity of biomass, and higher maximum growth were found in continuous cultivations compared with batch cultivations. The substrate utilization and the yield of biomass of this strain were much higher than reported for several other Thermus strains. Reproducibility of kinetic data seemed not to depend on the type of cultivation vessel, on the basis of the types of vessels tested, and instability of the population was not observed during cultivations. Production of extracellular protease in our cultivations was apparently growth associated in batch culture, and the specific rate of production in continuous culture was dependent of the dilution rate, implying that certain kinds of regulatory mechanism(s) might be involved.