Leaf samples of white clover were collected from 19 irrigated white clover (Trifolium repens) pastures in the south-west of Western Australia and tested for virus infection by enzyme-linked immunosorbent assay. Clover yellow vein virus (CYVV) was found in 16 pastures at infection levels of up to 23% and white clover mosaic virus (WCMV) in 10 at levels up to 83%. None of the white clover pastures with a high incidence of WCMV had been resown with white clover within the last 10 years, whereas those resown within the last 5 years had little or no infection. As previously reported in tests on different white clover pastures in the same irrigation area, widespread infection with alfalfa mosaic virus (AMV) and occasional infection with subterranean clover red leaf virus (SCRLV) was also found. Two or more viruses were found within 16 of the pastures with at least 3 having all 4 viruses. AMV and WCMV were detected in flatweed (Hypochaeris glabra) and AMV was detected in clustered dock (Rumex conglomeratus), both commonly occurring weeds in the pastures. In tests on the perennial ryegrass (Lolium perenne) component of 18 white clover pastures, infection with barley yellow dwarf virus was found in 14 at levels up to 5%. In addition, 11 of the pastures contained a virus which reacted with potyvirus-specific monoclonal antibodies, presumably ryegrass mosaic virus (RyMV), which was detected at levels up to 34%. Live aphids were trapped at 8 different times during 1995 in one pasture that was infected with WCMV, CYVV, AMV, and SCRLV. Blue-green aphid (Acyrthosiphon kondoi) and oat aphid (Rhopalosiphum padi) were the only species caught, both reaching peak populations in midwinter, but only the latter was found in summer. No virus transmission was detected when the aphids caught were fed individually on subterranean clover (T. subterraneum) indicator plants. It is concluded that WCMV poses a threat to the productivity of white clover within irrigated pastures, especially when present in mixed infection with AMV. CYVV is also commonly found but normally not at high enough incidences to pose a serious threat. RyMV may pose a threat to the productivity of the perennial ryegrass component within white clover-based pastures.