1) Metal tolerant seeds of Agrostis tenuis, Anthoxanthum odoratum and Plantago lanceolata were collected from the zinc/lead mines at Trelogan, North Wales, and non metal tolerant seeds were collected from three nearby sites with normal soils. 2) Seedlings were grown in competition experiments on normal soil in pure stands and 50:50 mixtures of normal and tolerant material. 3) Tolerant and non tolerant seedlings of each species were also grown in competition with S.23 perennial rye grass. 4) Data are given for mean plant dry weight, tiller or leaf number, and relative fitness of the tolerant material in pure stand and mixtures. 5) The non tolerant ecotype of each species had a greater dry weight and more tillers/leaves than the zinc/lead tolerant ecotype when grown in pure stands. The mean relative fitness in pure stand of the zinc/lead tolerant material was 0.75. The difference in dry weight and tiller/leaf number was considerably greater in mixed than in pure stands. The mean relative fitness of the zinc/lead tolerant material was 0.57. In mixed stands with ryegrass, the two grass species showed relative fitness values similar to those obtained in the single species mixture-A. tenuis 0.50 and Anthox. odoratum 0.56. P. lanceolata was scarcely affected by the presence of ryegrass, its mean relative fitness being 0.90. 6) Coefficients of selection were 0.5 and greater in relatively ordinary conditions.