The net assimilation rate (E) of Kleinwanzleben sugar-beet was the same as that of three types of wild sea-beet (Beta vulgaris subsp. maritima) when the leaf-area index (L) was near to I. In a subsequent period, when mean L of sugar-beet and of the leafiest wild beet type was 2.5, there was an inverse relation between E and L of the three wild types, and E of sugar-beet was then much greater than that of the wild type with equal L but was little different from that of the wild type with smallest L (about 1.5). It is concluded that the development of sugar-beet from its wild ancestors by selection and breeding has not affected the intrinsic photosynthetic efficiency of the leaves, but has diminished the effect of mutual-interference between leaves, so that E falls less rapidly as L increases, i.e. it has decreased the leaf-density dependence of E. This change may be related to the difference in form between sugar-beet and wild beet plants.