Mycorrhizal effectiveness depends on the compatibility between fungus and plant. Therefore, genetic variation in plant and fungal species affect the effectiveness of the symbiosis. The importance of mycorrhizal plant and mycorrhizal fungus origin was investigated in two experiments. In the first experiment, clones (cuttings) of Salix repens L. from three different origins (two coastal dune and one inland ecosystem) were inoculated with three mycorrhizal fungi (the ectomycorrhizal fungi Hebeloma leucosarx and Paxillus involutus; the arbuscular mycorrhizal fungus Glomus mosseae) on two soils with different nutrient availabilities. In the second experiment, homogeneous plant material (one clone of S. repens) was inoculated with four isolates of H. leucosarx, all found in association with S. repens. Plant origin had a large effect on symbiotic effectiveness, assessed by three different criteria (shoot biomass, shoot N-content, shoot P-content). This effect was probably mediated through genetic differences in root parameters. There was also a large plant origin×soil type interaction, with the plants from the nutrient-poor habitat performing better on the poorest soil. The plants from the more nutrient-rich habitat performed better on the relatively rich soil. Effects of plant origin on symbiotic effectiveness were also fungal-species specific. Fungal origin had only a minor effect on symbiotic effectiveness. Studies concerning the ecological significance of mycorrhizal symbiosis should be more explicit about both mycorrhizal plant and fungus origin and compatibility.