Cerato-ulmin (CU) presence was monitored on cell surface and quantitatively determined in mycelial extracts of the elm pathogens Ophiostoma novo-ulmi (races EAN and NAN) and O. ulmi and of the non-pathogenic O. piceae. CU was detected on the surfaces of Ophiostoma novo-ulmi (races EAN and NAN) and, for the first time, of the weak Dutch elm disease pathogen O. ulmi and the non-pathogen O. piceae. Quantitative determination of CU content in the mycelial extracts of the three species showed that high CU cellular content is associated with high CU content in culture filtrates. The content of CU in biomasses and in culture filtrates was influenced by temperature, growth phase and fungal species or race. CU synthesis occurred during the stationary phase and in the late logarithmic phase when fungi were grown at 23° and 32°C, respectively. High temperatures of growth (32°) did not have a negative effect on the cellular CU content but severely hampered CU secretion in high CU-producers O. novo-ulmi isolates.