The leaf-area index (leaf area per unit area of land, L) of field crops of kale and sugar-beet was varied experimentally by removing different fractions of the plant population distributed uniformly through the crop. The net assimilation rate (E) was determined in subsequent periods of 10–14 days. For kale, E decreased nearly linearly with increase of L throughout the range from I to 5. E of sugar-beet was less affected by change in L and was apparently not decreased until L rose above about 3. Because of this dependence of E on L, the rate of dry-matter production per unit area of land, or crop growth-rate (C=EL), showed a curved relation to L; for kale it increased to a maximum when L was between 3 and 4 and fell again at higher values of L. Maximal C for sugar-beet occurred beyond the range of L tested, probably between L=6 and L=9. This optimal L for dry-matter production by sugar-beet crops probably lies near the upper limit of the current agricultural range, so there is little, if any, scope for increasing the dry-matter yield by further increase in L. For heavy kale crops L is already far in excess of the optimum, and it may be possible to increase the total dry-matter yield of kale by repeated thinning or defoliation to hold L near the optimum.