The responses of Chenopodium album L. and Senecio vulgaris L. to inter-and intra-specific comjjetition were investigated in both additive and replacement series experiments under glasshouse conditions. When grown with tomato the two weed species had similar effects on shoot dry weight at low densities but S. vulgaris showed more competitive effect at higher densities. Weed density did not affect the concentrations of N. P. K, Ca or Mg in tomato shoots but dry matter and total amounts of nutrients were reduced increasingly with the increase in density of both weeds. The concentration of nutrients in the shoots of the weeds was not affected by density but dry matter yield and total nutrient accumulation per plant fell as the density increased. The concentrations of N, P, K and Mg in the shoots were higher in C. album than S. vulgaris but that of Ca was lower. In a replacement series experiment the two weed species behaved differently. With C album, both dry matter yield and total nutrient per plant were reduced as its proportion in the mixture increased but, with S. vulgaris. dry matter per plant increased with its proportion in the mixture. One plant of C. album grown with five S. vulgaris gave a higher shoot dry weight than when present in higher proportions or in a •Present address: