The temperature-growth relationships of a range of isolates representing the recently identified OPC and OPH sibling species within Ophiostoma piceae were studied. Most OPC isolates grew significantly faster than OPH isolates on 2% Oxoid malt extract agar (MEA) between 17·5 and 27·5 °C, but between 27·5° and 30° growth rates of all OPC isolates studied declined rapidly in contrast to those of OPH isolates. The upper limit for growth of OPC isolates of MEA was ca 35°, whereas that for OPH isolates was ca 31°. A simple growth test at 32° on MEa will discriminate between most isolates of the two species, and can be used as a supplementary aid to their identification.