Over-summering grasses were collected in the south-west of Western Australia in 1991 and 1992 and tested by ELISA using serotype specific polyclonal antibodies for presence of barley yellow dwarf virus (BYDV) serotypes MAV, PAV and RPV (1991) or MAV, PAV, RPV and RMV (1992). In 1991, 33 samples from 33 sites were tested and MAV was detected in four of 12 samples of Pennisetum clandestinum found infected with BYDV. Presence of MAV was confirmed by retesting these samples using MAV specific monoclonal antibodies. In 1992, 802 samples from 16 grass species were collected from 579 sites in six regions. BYDV was detected in 214 (27%) samples at 178 (31%) sites. MAV was found in 50% and RMV in 38% of infected samples. Both were found either alone or in mixed infections with each other or with PAV and/or RPV; all four serotypes were found in 18 (8%) infected samples. The most important hosts were four perennials: Cynodon dactylon, Eragrostis curvula, Paspalum dilatatum and P. clandestinum. Eight other perennial and five annual grass species were also infected. MAV was most commonly found in C. dactylon and P. clandestinum and RMV in P. dilatatum. All four serotypes were present in the six regions sampled, but the relative proportions of the serotypes found varied from region to region. This paper represents the first extensive survey of MAV and RMV serotypes of BYDV in over-summering grasses in Australia. Aphid species found during this survey infesting over-summering grasses were Rhopalosiphum padi, R. maidis and Hysteroneura setariae. In the Mediterranean type climate of the south-west of Western Australia, BYDV and aphids on over-summering perennial grasses constitute the main reservoir of infection and means of spread to cereal crops. Factors favouring grass survival over summer result in increased cereal crop infection.