An aerobic, thermophilic, halotolerant and Gram-positive bacterium, designated strain C5, was isolated from a high-temperature oil field, located in Sfax, Tunisia, after enrichment on tyrosol. Strain C5 grew between 25 and 70 degrees C and optimally at 50 degrees C. It grew in the presence of 0-12% (w/v) NaCl, with optimum growth at 3% (w/v) NaCl. Strain C5 was able to degrade tyrosol aerobically, in the presence of 30 g L(-1) NaCl and under warm conditions (55 degrees C). The degradation of tyrosol proceeded via p-hydroxyphenylacetic and 3,4-dihydroxyphenylacetic acids. The products were confirmed by HPLC and GC-MS analyses. Strain C5 was also found to degrde a wide range of other aromatic compounds, including benzoic, p-hydroxybenzoic, protocatechuic, vanillic, p-hydroxyphenylacetic, 3,4-dihydroxyphenylacetic, cinnamic and ferulic acids, phenol and m-cresol. Moreover, strain C5 was grown on diesel and crude oil as sole carbon and energy sources. Strain C5 was also able to utilize several carbohydrates. Phenotypic characteristics and phylogenetic analysis of the 16S rRNA gene sequence of strain C5 revealed that it was related to members of the genus Geobacillus, being most closely related to the type strain of G. pallidus (99% sequence similarity). In addition, we report on growth of the type strain of G. pallidus on different aromatic compounds and hydrocarbons.