Extracellular laccase production of the Dutch elm disease pathogens Ophiostoma ulmi and O. novo-ulmi was monitored during growth in vitro. Under conditions conducive to mycelial growth, O. novo-ulmi secreted appreciable amounts of laccase (0·26 ± 0·06 U ml−1) in parallel with growth whereas laccase activity in O. ulmi was barely detectable (⩽ 0·02 U ml−1). In contrast, yeast phase cells of both Dutch elm disease fungi produced little or no detectable laccase. These findings were not significantly influenced by the temperature. For O. novo-ulmi laccase production was significantly affected by pH. Levels of laccase were 2·5 fold lower at pH 4·0 and 7·0 compared with pH 5·5 (P