Plasmodiophorid parasites in the genus Polymyxa infect roots by means of zoospores and transmit more than 15 soil-borne viruses in a wide range of arable crops. Barley mutants, selected for variations in root hair formation and morphology, were used to demonstrate that root hairs were important but not essential for infection by zoospores of Polymyxa graminis. The relative rates of parasite establishment in roots were determined indirectly as the relative number of zoospores released by roots inoculated with P. graminis in wild-type and mutant plants. The number of P. graminis zoospores released per gram root fresh weight was significantly reduced in brb and rhl1.b mutants, both of which have no root hairs. This is an important result because there are no natural sources of resistance to P. graminis. Reducing infection levels of viruliferous P. graminis will slow the build up of virus inoculum in the soil and the selection of strains able to overcome the virus resistance in current cereal cultivars.