Abstract: Thermophilic gram-positive anaerobic host cells, for example Thermoanaerobacterium saccharolyticum (“T sacch”), express heterologous biomass degrading enzymes, such as cellulases, and are able to produce useful fermentation products from cellulose. Useful fermentation products include, for example, ethanol, acetic acid, lactic acid or CO2. In order to provide maximum expression and activity levels, biomass degrading enzymes can be expressed from codon-optimized nucleotide sequences, can be expressed under the control of a high-efficiency promoter, and/or can be fused to a signal peptide. In addition, the host cell, for example, a T sacch host cell, can be genetically altered to further improve ethanol production, for example by disrupting the production of organic products other than ethanol.