The existence of deep-sea hydrothermal thermophilic archaebacteria raises interesting questions about their metabolism and metabolic products both in a natural environment and in a culture medium. Moreover, the use of these microorganisms as a potential source of molecules of biotechnological interest must be considered, in the light of the fact that metabolic produits can enhance the risk of microbially-induced corrosion. Organic acids formed by an anaerobic thermophilic sulphur-dependent archaebacterium were analysed as tert-butyldimethylsilyl derivatives by GC and GC/MS. Data indicated the presence in significant amounts of propanoic, 2-methyl propanoic, 3-(methylthio) propanoic, phenylacetic and 4-hydroxyphenylacetic acids. In addition, the use of stable-isotope labelled amino acids demonstrated specific pathways for 3-(methylthio) propanoic and phenylacetic acids.