Various aspects of cell proliferation have been examined in the stele and in the cortex-epidermis between 50 and 800 μm basal to the junction between the cap initials and the rest of the meristem, in newly emerged, 0.2 and 4 cm long lateral roots of Viciafaba following a i-h exposure to tritiated thymidine. Cell proliferation was found to differ in the two tissues in a number of ways, e.g. cell doubling time was usually shorter and the growth fraction generally larger in the cortexepidermis than in the stele. Cell proliferation changed basally along each tissue, especially in the relative and absolute sizes of the various meristematic and quiescent cell fractions and in cell doubling and mean cycling times. It was concluded that the apical meristem expanded longitudinally as the lateral roots lengthened from newly emerged to 4 cm. In terms of cell doubling time, although no change took place in the main site of cell proliferation in the cortex-epidermis, that in the stele shifted basally along the root as the apical meristem expanded.