The relative competitive abilities ofCalamagrostiscanadensis(Michx.) Beauv. andEpilobiumangustifoliumL. were tested in two sets of replacement series experiments. Both species were grown in monocultures and a range of mixtures in 25-cm pots. In the first set, substrates were held at either 9 or 21 °C; in the second set the pots were fertilized at high or low rates. In the 21 °C treatmentC.canadensiswas more competitive thanE.angustifolium(relative crowding coefficient forC.canadensistowardsE.angustifoliumwas 2.88), while there were little competition differences in the cool soil conditions. Under the cool soil temperatures, however,E.angustifoliumshowed higher vegetative reproduction than under the warm soil conditions. In the high nutrient conditions,C.canadensiswas more competitive thanE.angustifolium(relative crowding coefficient forC.canadensistowardsE.angustifoliumwas 5.84). There was little competition in the low nutrient experiment. These experiments indicate that if both species colonize a site simultaneously,C.canadensiswill outcompeteE.angustifoliumunder most conditions, as suggested from field observations of earlier researchers.