In tests on seed samples from 26 commercial seed stocks of lucerne (Medicago sativa) to be sown in south-western Australia in 200 1, infection with Alfalfa mosaic virus (AMV) was found in 21 and Cucumber mosaic virus (CMV) in 3 of them. Bean yellow mosaic virus (BYMV) and Pea seed-borne mosaic virus (PSbMV) were not detected in any. Incidences of infection within individual affected seed samples were 0.1-4% (AMV) and 0.1-0.3% (CMV), and the infected seed stocks were from 3 (CMV) and at least 11 (AMV) different lucerne cultivars. In a survey of 31 three-year-old lucerne pastures in the same region in 2001, in randomly collected samples, AMV was found in 30 and luteovirus infection in 11 pastures. Pastures in high, medium, and low rainfall zones were all infected. Incidences of AMV within individual infected pastures were high, with 50-98% of plants infected in 20 of them and only 3 having <10% infection, but luteovirus incidences were only 1-5%. In addition to various cultivar mixtures, at least 8 (AMV) and 3 (luteoviruses) different individual lucerne cultivars were infected. When the species of luteovirus present were identified, they were Bean leaf roll virus, Beet western yellows virus (= Turnip yellows virus), or Subterranean clover red leaf virus (= Soybean dwarf virus). CMV and legume-infecting potyviruses (BYMV, PSbMV, and Clover yellow vein virus) were not detected in any of the lucerne samples. Acyrthosiphon kondoi infestation was common in the samples collected, and A. pisum and Aphis craccivora were also found. Widespread infection in lucerne stands, and their frequent colonisation by aphid vectors, are cause for concern not only because of virus-induced production losses in lucerne itself but also because they provide virus infection reservoirs for spread to nearby grain legume crops and annual legume pastures.