SUMMARYIn a series of four pot experiments single plants of kale and sugar beet or wheat were grown alone and with sixteen plants of several different weed species for from 7 to 11 weeks. Two levels of nitrogen were included.The effects of different weeds on dry‐weight growth of the crop plants were used as a measure of their competitive abilities. The results from each experiment were corrected on the basis of the effects ofChenopodium album, which was included as a standard in all experiments. The order of competitive abilities obtained was: with kale, firstSinapis arvensis, thenPolygonum convolvulusandAlopecurus myosuroides, thenChenopodium album, Polygonum uviculureandP. persicaria, followed byMatricaria maritimassp.inodora, and finallyStellaria media, Capsella bursa‐pastoris, Veronica perszcaandAnagallis arvensis;with wheat, a similar order, butPolygonum convolvulushad less competitive ability thanAlopecurusand about the same asChenopodiumandP. aviculure, andMatricariahad about the same value asVeronica.The effect of nitrogen on kale and on sugar beet was statistically significant, but on wheat it was significant in only one experiment. Weed effects generally were not significantly decreased by high nitrogen; the effects ofSinapison kale, and ofChenopodiumon wheat were increased, The leaf nitrogen percentage of kale and sugar beet was greatly increased by high nitrogen, but usually only slightly affected by weeds; nitrogen deficiency seems not to have been the sole cause of the weed effects. In two experiments with wheat, although high soil nitrogen increased the leaf nitrogen percentage of plants with weeds, it had little effect on their leaf dry weight, so that competition for nitrogen appeared non‐effective. In a third experimentAlopecurusandSinapisboth appeared to compete with wheat for nitrogen.In spite of the greater number of weeds per pot, their yields were usually decreased by adding a single indicator plant to their pots. Those weeds were least affected which themselves most reduced the growth of the indicators.The method used is judged to give results similar to those that might be found in the field, and to have advantages over alternative methods for estimating the relative importance of weeds as competitors of