SUMMARYThere was a significant inverse correlation (P= 0=001) between concentrations of mushroom viruses 1 and 2 in sporophores and amounts of mycelial growth on malt agar of isolates taken from them. Increasing virus concentrations decreased linear growth of one mushroom strain from 76 mm (healthy) to 35 mm (mildly infected) and 7 mm (severely infected) when incubated at 25°C for 3 wk.Mycelial growth rates of isolates from healthy and from virus‐infected mushrooms were compared on eleven agar media. All media clearly differentiated between healthy and severely infected isolates, but fewer separated healthy from mildly infected isolates. Those that did contained maltose, sucrose or starch as carbon source. Media containing peptone usually gave better differentiation than those with other sources of nitrogen, but the best differentiation was obtained with malt agar.Growing healthy and infected isolates on a range of media affected their subsequent growth on malt agar, the growth of some isolates apparently being changed permanently after 2 months on some of the different media. Whereas none of the infected isolates grew less rapidly after this treatment, the growth of some of the mildly infected isolates improved to such an extent that they could no longer be distinguished from healthy isolates.After heat‐treatment (1–6 wk at 33°C), mycelial growth rates of infected isolates were increased, but viruses 1 and 2 were not always eliminated unless the heat‐treatment was begun immediately after subculture.Mycelial growth rate and colony characters are not infallible criteria of the presence or absence of virus, a feature of particular significance when checking the health of mushr