This study determined whether the tolerance of yellow lupin to waterlogging, observed in experiments in controlled environments, occurs under field conditions. Of particular interest is the impact of waterlogging on the distribution of roots because lupin is exposed to terminal drought in the south of Western Australia, which in itself can have a profound effect on yield. A field experiment was undertaken in the central grain-growing region of Western Australia near Beverley using hydraulically isolated plots to impose and remove waterlogging in a duplex soil. The responses of root and shoot growth of narrow-leafed and yellow lupin to waterlogging in the field were similar to those observed in the controlled environment experiments. In the field experiment, waterlogging had no effect on seed yield of yellow lupin but reduced it by 61% in narrow-leafed lupin. Waterlogging more than halved the dry weight of narrow-leafed lupin but reduced it by only 19% in yellow lupin. In yellow lupin, yield was 3.4 t/ha with waterlogging and 3.8 t/ha without waterlogging, compared with 1.4 t/ha with waterlogging and 3.5 t/ha without waterlogging in narrow-leafed lupin. Waterlogging had no effect on the harvest index of yellow lupin (0.26) but reduced it from 0.36 to 0.31 in narrow-leafed lupin. The larger effect of waterlogging on the yield of narrow-leafed lupin was mainly attributable to fewer pods.Net root growth ceased during waterlogging in both species. After waterlogging, roots of yellow lupin grew at a similar rate to the controls, whereas roots of narrow-leafed lupin grew at a much slower rate than the controls. Waterlogging halved the root density of yellow lupin at 25 cm depth and almost eliminated the roots of narrow-leafed lupin at this depth. After waterlogging, root production in the surface 10 cm increased to about 0.5 cm/cm(3) in yellow lupin but to 0.2 cm/cm(3) in narrow-leafed lupin. At depth (> 20 cm), roots of waterlogged yellow lupin continued to grow while those of waterlogged narrow-leafed lupin grew little, if at all. Yellow lupin tolerated waterlogging in the field better than narrow-leafed lupin because it re-established its root system at depth after waterlogging was removed and it produced more fertile pods.