Cultures of Pythium sp. in vitro were inoculated with a sterile concentrated suspension of Tobacco Mosaic Virus (TMV). The cultures were shaken continuously. Virus could be demonstrated in the mycelia beginning with the seventh day after inoculation.Virus-containing mycelia were subcultured in virus-free media for a period up to 2 months and tested regularly for virus content.Virus was still present in the mycelium after 2 months of subculture, but the concentration was low. After growth on solid medium, infected mycelium still contained virus, an indication that the virus is able to persist in the mycelium. When hyphal tips were used for subculturing the cultures eventually became virus-free. It is likely that virus particles lag behind the growing hyphal tips because of the rapid mycelial growth.