During the summer periods of 2000 and 2001, incidences of infection with Barley yellow dwarf virus (BYDV) and Cereal yellow dwarf virus (CYDV) were determined in grass weeds and volunteer cereals surviving at isolated sites throughout the grainbelt of south-western Australia, which has a Mediterranean-type climate. Samples of Cynodon dactylon, Eragrostis curvula, Erharta calycina, Pennisetum clandestinum, and volunteer cereals (mostly wheat) were tested for BYDV (serotypes MAV, PAV and RMV) and CYDV (serotype RPV), and those of at least 19 other grass species were tested for BYDV only ( serotypes PAV and MAV). In 2000, BYDV and/or CYDV were detected in 33% of 192 sites in 0.7% of 26 700 samples, and in 2001 the corresponding values were 19% of 176 sites and 0.5% of 21 953 samples. Infection was distributed relatively evenly throughout the different annual average rainfall zones of the grainbelt, but when sites were categorised according to actual rainfall for late spring to early autumn, the proportion of sites and samples infected increased where such rainfall exceeded 300 mm. In both summer sampling periods, the most abundant grass species were C. dactylon and E. curvula, with BYDV and/or CYDV being detected in 0.1-0.6% and 0.1-0.5% of samples, respectively. The corresponding incidences were 0-1% for Erharta calycina, 7-8% for P. clandestinum, and 0.2-2% for volunteer wheat. The most abundant species tested for BYDV only were Chloris truncata and Digitaria sanguinalis, with infection incidences of 0.2-0.7 and 0.2-0.3%, respectively. Chloris virgata (2-3%) and Urochloa panicoides (0.3-0.6%) were the only other infected species. Within individual sites and host species, the greatest incidences of CYDV were in P. clandestinum (23% in 2000 and 18% in 2001) and of BYDV in Chloris virgata (14% with PAV and 12% with MAV in 2000). Small populations of grass-infesting aphids were found over-summering at 26% ( 2000) and 3% ( 2001) of sites and occurred in all 3 annual rainfall zones. The predominant species was Hysteroneura setariae, but Rhopalosiphum maidis, R. padi, and Sitobion miscanthi occurred occasionally. Presence of oversummering BYDV, CYDV, and aphids in all rainfall zones has important implications for virus spread to cereal crops throughout the grainbelt.