Fifteen yellow-pigmented halotolerant Thermus strains from shallow marine hot springs on a beach on S. Miguel, Azores, several isolates from terrestrial hot springs on S. Miguel and mainland Portugal, and four reference strains were subjected to a numerical taxonomy study based on 61 characters by unweighted average linkage analysis (UPGMA) using the simple matching (SSM) coefficient. This analysis produced four clusters at the 75 % similarity level. Two clusters, A and B, were composed of halotolerant strains from the beach hot springs and one strain isolated inland, all of which also grew at 80 "C; cluster C was composed of strains from one hot spring in mainland Portugal, while cluster D was composed of terrestrial strains from the Furnas area of S. Miguel. 'T. thermophilus' HB-8 and Thermus B were also halotolerant, also grew at 80 "C, and formed a large group at the 61% similarity level with the strains of clusters A and B.