Thirteen strains of Thermus isolated from New Zealand hot pools were screened for their ability to grow on minimal or near-minimal media and to exhibit extracellular protease activity. Environmental regulation of protease production by one such strain, Thermus Ok6, was investigated in detail. Protease activity was maximally derepressed during growth under conditions which lead to energy starvation (i.e. highly oxidized carbon substrate, oxygen limitation, slow growth rate and a high external pH), and was variably repressed by different nitrogen sources. These results suggest that the function of the enzyme is to scavenge carbon/energy and nitrogen. The protease was loosely attached to the cell under these growth conditions but could be released by growing the organism in high concentrations of trypticase peptone plus yeast extract, by exposing the culture to moderate shear forces or by washing cells in high ionic strength solutions of various inorganic salts, organic acids or amino acids. A simple and rapid procedure for the partial purification of the enzyme is described.