Seed of 31 named cultivars and 11 other genotypes of Trifolium subterranean (subterranean clover) was sown in 3 major and 3 smaller scale field experiments. In these, bean yellow mosaic virus (BYMV) was spread from infector plants to test rows by naturally occurring aphids. The numbers of clover plants developing BYMV symptoms in each row were counted at different times. Based on the results from at least 2 field experiments, different genotype were assigned to 1 of 5 BYMV infection categories (A-E), ranging from greatest (A) to least (E) infection. Category A was found to contain genotypes that were latest maturing while, except for cv. Rosedale (ssp. brachycalycinum) which was in E despite being of intermediate maturity, category E contained earliest maturing genotypes. Except for cv. Rosedale when the percentages of plants with BYMV symptoms in the 3 main experiments were plotted against days to flowering previously established for each genotype the two were closely related. However, when young plants of cultivars in categories D or E were aphid-inoculated with 2 BYMV isolates, they became infected to a similar extent to young plants; of 2 late-maturing cultivars from category A. This suggests that except for cv Rosedale the extent of BYMV infection obtained in the field was determined by plant Physiological age plants of late-maturing genotypes being easier for viruliferous aphids to infect with BYMV than plants of early-maturing genotypes of the same age but physiologically older, a phenomenon known as 'mature plant resistance'. When plants of cv. Rosedale were aphid-inoculated with 7 different isolates of BYMV a systemic hypersensitive reaction developed with 5 isolates but not with two others. This suggests that cv. Rosedale has 'strain specific hypersensitive resistance' to BYMV. No systemic hypersensitive reaction developed when the same 7 BYMV isolates were aphid-moculated to cv. Dwalganup and named selections Pink Flowered and Northam Early, genotypes previously reported to react hypersensitively to potyvirus infection. The field screening procedure employed is suitable for use by T. subterraneum breeding programs seeking to develop new cultivars with resistance to BYMV. Cultivar Rosedale may prove a suitable source of resistance to use as a parent in crossing programs aimed at producing cultivars with natural BYMV resistance.