The yield losses caused by necrotic and non-necrotic strains of Bean yellow mosaic virus (BYMV) in narrow-leafed lupin (Lupinus angustifolius) were quantified in field experiments. Clover plants infected with either were introduced into plots to provide infection sources, and aphids spread infection to the lupin plants. When the effects of virus infection were examined in individual lupin plants infected with necrotic BYMV, they were killed by early infection so there was no seed production. With late infection, shoot dry wt, seed yield, and seed number were decreased by at least 55%, 80%, and 74%, respectively. With non-necrotic BYMV, shoot dry wt, seed yield, and seed number diminished with increasing duration of plant infection, these decreases ranging over 27-88%, 48-99%, and 35-98% for late to early infection, respectively. In partially infected stands in which both necrotic and non-necrotic BYMV were spreading, an additional incidence of 28% in plots with introduced non-necrotic strain foci over that in plots without introduced foci was sufficient to decrease overall seed yield significantly. However, an additional incidence of 10% was insufficient to do so in plots with introduced necrotic strain foci. In plots into which different numbers of clover plants infected with non-necrotic BYMV were introduced, subsequent incidence of infection depended on the magnitude of the initial virus source, and yield was decreased by 21-24%, 31-43%, and 64-66% with 4, 8, or 16 foci/plot, respectively. With both types of strain, yield loss in infected plants was mainly due to failure to produce any seed or to fewer seeds being produced, but smaller seed size also contributed. These results show that non-necrotic strains of BYMV have considerable yield-limiting potential in narrow-leafed lupin crops despite causing milder symptoms than necrotic strains. No evidence was obtained of seed-transmission of non-necrotic BYMV in narrow-leafed lupin, but a 0.2% seed transmission rate was detected in yellow lupin (Lupinus luteus).