The competitive ability of a tetraploid, and its morphologically indistinguishable progenitor diploid Dactylis glomerata L., from Galicia (Spain), was tested in a two-year study, using isolated plants and the same individuals grown at two densities (16 and 36 plants per pot, respectively). Inverse linear regression equations were used to quantify the response of the different ploidy levels (hereafter cytotypes) to density. For the diploids, inter-cytotype competition coefficients were significantly higher than intra-cytotype coefficients. The substitution rate increased from 1. in the first year to 3.9 in the second year, when most of the diploids failed to flower and some died. The competitive superiority of the tetraploids was associated with higher mean tiller weight, particularly in the mixed cultures. The tetraploids also had heavier seeds and faster leaf production in early spring, and they flowered earlier than the diploids. Several of these differences between cytotypes appear to be due to differences in ploidy level and may play a major role in the establishment of the tetraploids in Galician pastures, where competition is high.