Ophiostoma quercus is an ophiostomatoid fungus strictly related to the Ophiostoma's (O. ulmi, O. novo-ulmi, and O. himal-ulmi) that cause Dutch elm disease (DED). O. quercus has a number of morphological characteristics in common with the DED pathogens, and is a well-known and economically important sapstaining fungus occurring worldwide on hardwoods and commercially produced pines, and causes typical cankers on oak stems. In elm trees O. quercus can survive for months without causing any disease symptoms. DED fungi produce cerato-ulmin (CU), a class II hydrophobin, which is generally considered as the main toxin potentially involved in various phases of the DED pathogenesis. In the present work we isolated and sequenced the orthologue of the cu gene in the O. quercus isolates H988, H1042, and H2053. Moreover the CU protein from O. quercus isolate H988 was also purified and characterized. Sequence analysis showed that there is a pronounced difference between the whole cu gene region of O. quercus and the homologous fragments of the DED-causing species O. ulmi, O. novo-ulmi, and O. himal-ulmi. It also appeared that differences in the structural conformation of the promoter were unlikely to play a role in the modulation of the transcript level and that, for O. quercus, differences in CU production did not result from the potential different regulation levels. Clear differences were shown in the transcriptional unit of the cu genes and in the amino acid sequences among all the CUs. The purified O. quercus CU was separated using matrix-assisted laser desorption ionization/time of flight (MALDI-TOF) spectrometry into seven forms of increasing molecular weight from 7190 to 7724Da. The hydrophobicity profiles indicated that two regions of the O. quercus CU protein were more hydrophobic than the corresponding regions of the CUs of the DED fungi. The O. quercus CUs had theoretical isoelectric point values similar to those of the DED fungi. Finally, the contradiction between the consistent differences between these four Ophiostoma species in the cu gene region and in the CU proteins and their strict phylogenetic relationship is discussed.