AbstractThree experiments were carried out in boxes to determine the response ofL. perenneseedlings to competition fromP. trivialisseedlings at different densities and ages and when the proportions of the two species were varied according to the replacement principle. Tiller numbers per plant were significantly reduced byP. trivialisat densities commonly found in the field. Prior establishment ofP. trivialiscaused disproportionate damage toL perennetillering and the early removal of such plants resulted in yield benefits toL. perenne.When sown together,P. trivialiswas shown to be competitive towardsL. perenneduring the first 4 weeks of establishment but during the next 4 weeks, although competition was still occurring between the two species to the detriment ofPoa, relative productivity (but not total productivity) was marginally greater in the mixture. The need for early herbicide treatment to prevent interspecific competition between graminaceous species during establishment is discussed.