Polyamines of thermophilic archaebacteria were analysed by high-performance liquid chromatography and gas chromatography. Thermoplasma acidophilum and Thermoplasma volcanium ubiquitously contained spermidine and spermine. Four species of Sulfolobus, S. acidocaldarius, S. solfataricus, S. metallicus and S. shibatae, two species of Acidianus, A. brierleyi and A. infernus, and Metallosphaera sedula, contained norspermidine and norspermine in addition to spermidine and spermine, but quantitative distribution profiles were species-specific. A tertiary tetra-amine, N4-aminopropylspermidine, and a quaternary penta-amine, N4-bis(aminopropyl)spermidine, were detected as a major polyamine in three species of Thermococcus, T. celer, T. litoralis and T. stetteri, and two Pyrococcus species, P. furiosus and P. woesei. This is the first report on the occurrence of branched polyamines in archaebacteria.