Under conditions of natural cucumber mosaic virus (CMV) spread, eight alternate host species found associated with Lupinus angustifolius (narrow-leafed lupin) and/or Trifolium subterraneum (subterranean clover) were infected commonly and another nine sporadically. Five of these were new records. Because seed of herbaceous plant hosts provides a possible route for virus persistence through dry summer conditions, CMV seed transmission was tested for in alternative hosts. Seed of seven species systemically infected following sap inoculation was tested, but CMV seed transmission was only detected in M. polymorpha (0.7%) and M. indica (0.1%). When seed of 14 potential alternative host species that became systemically infected through natural virus spread was tested, CMV seed transmission was found only in C. decumbens (0.5%). No CMV was detected in Citrullus lanatus growing as a deep-rooted, herbaceous summer weed following CMV-infected L. angustifolius crops, or in the perennial Acacia saligna growing adjacent to a previously CMV-infected L. angustifolius field. CMV persisted through seed transmission over summer for up to 5 years in grazed, self-regenerated T. subterraneum swards. It is concluded that under the conditions of broadacre agriculture, in the Mediterranean-type climate of Western Australia, weed hosts are unlikely to be an important means by which CMV persists over summer, but seed transmission in naturalized M. polymorpha and C. decumbens may occassionally play a minor role. Moreover, despite being seed-borne in T. subterraneum, CMV did not persist readily enough from year to year in grazed swards for T. subterraneum pastures to play more than a minor role as a CMV source for infection of L. angustifolius.