Nine strains of Aspergillus were isolated from desert soils in Iraq. The thermotolerant Aspergillus niveus RMF 7883 was found to possess a highly active cellulolytic system. Filter-paper degrading activity was 14·5 IU ml −1 whilst the activities of β-glucosidase and endoglucanase were 37 and 326 IU ml −1 respectively. The enzyme degraded cotton in 2–4 h and was stable for 5 months at −18° and +4°. This organism was a hyperproducer of β-glucosidase in comparison with Trichoderma reesei Rut C30, and appears to be a promising strain for the production of a highly active, thermotolerant and stable cellulolytic preparation.