Fungal expression systems provide a promising, industrially-relevant alternative to bacteria for heterologous enzyme production. We are developing the yeast Kluyveromyces lactis and the filamentous fungus trichoderma reesei for the extracellular production of thermophilic enzymes for the pulp and paper industry. Gram amounts of largely pure xylanase A from Dictyoglomus thermophilum have been produced in K. lactis chemostat culture. D. thermophilum xynB gene has been reconstructed according to T. reesei codon preferences and substantial amounts of enzyme produced in T. reesei. A heterologous fungal gene, Humicola grisea xyn2 was expressed in T. reesei without codon modification. Initial amounts of the XYN2 protein were of a gram per liter range in shake flask cultivations and the gene product was processed by the heterologous host in a similar manner to its endogenous xylanases.