Dry weight production of seed-derived material of eight genotypes from each of two populations of Poa annua, one from an open and the other from a closed habitat, was compared as pure stands and in mixture with Lolium perenne cv. S23. Comparisons were made in a glasshouse experiment after 13 weeks growth, when uncut, and when cut to a height of 1.5 cm at 5, 7, 9, II and 13 weeks from planting- Significant differences were found between genotypes within both populations when cut in pure stands and in mixture with Lolium, and also in uncut stands in mixture with Lolium. When uncut and in pure stands inter-genotypic differences were found only in the open population. Between population differences in within population variation were significant only when the populations were cut and in mixed stands, the open population being more variable. The closed population was significantly more variable when uncut and in mixed stands than in any other treatment combination, whilst the open population did not vary significantly with treatment. The occurrence of genetically-based inter-population differences in competitive ability in P. annua is confirmed.