The present invention is directed to cellulytic host cells. The host cells of the invention expressing heterologous cellulases and are able to produce ethanol from cellulose. According to the invention, host cells expressing a combination of heterologous cellulases can be used to produce ethanol from cellulose. In addition, multiple host cells expressing different heterologous cellulases can be co-cultured together and used to produce ethanol from cellulose. Furthermore, the invention demonstrates for the first time the ability of Kluveryomyces to produce ethanol from cellulose. The yeast strains and co-cultures of yeast strains of the invention can be used to produce ethanol on their own, or can also be used in combination with externally added cellulases to increase the efficiency of saccharification and fermentation processes.