Cellulose, is an unbranched glucose polymer composed of D-glucose units linked by 1,4--D-glucosidic bonds. It is one of the most abundant biopolymers on Earth and represents an important renewable energy source. Genes encoding endoglucanases are widely distributed among fungi and Bacteria; recently one was identified in the Archaeon Pyrococcus furiosus suggesting the occurrence of polysaccharides in hydrothermal vent environments (1). We have identified a DNA fragment from Sulfolobus solfataricus Gstrain containing an ORF encoding a putative endoglucanase (CelSs) of 322 aminoacids (about 36 kDa) with significant homology to the Thermotoga maritima and T. neapolitana endo-1,4--glucanase. The gene was demonstrated to be transcribed in vivo and the transcription start site was identified by primer extension. The cellulase activity was detected in the supernatant of S. solfataricus G cultures and specific enzyme staining after SDS-PAGE revealed the presence of two proteins with apparent molecular mass of about 43 kDa and 36 kDa. The insertion into suitable vectors for the high level expression of the celSs open reading frame in E.coli and the characterisation of the recombinant protein are underway.