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Populations of Dactylis glomerata from Norway and from Portugal were grown in pure stands and in 50:50 mixtures under natural conditions in Aberystwyth. Stands were sown at monthly intervals between September 1969 and September 1970. Monthly harvests from each sowing were taken either from stands which were defoliated regularly or from stands with uninterrupted growth from sowing. In both managements the environmental conditions during establishment of the stands had a major influence on the subsequent relative competitive abilities of the two populations. This was most pronounced for the September sowings in which the Portuguese population was more aggressive initially and this aggressiveness was maintained throughout the following summer even though environmental conditions at that time did not favour this population. Similarly, the Norwegian population was more aggressive in stands sown in May and June. Again this aggressiveness often was maintained throughout the series of harvests even though this population was dormant during the winter. In other monthly sowings the initial competitive relationships were modified by the effect of the subsequent environmental conditions on the seasonal patterns of growth in the two populations. These reversals of aggressiveness were more marked in stands which were defoliated regularly than in those with uninterrupted growth. [ABSTRACT FROM AUTHOR]