Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings responded differently regarding rate of photosynthesis when inoculated with three different ectomycorrhizal fungi. Rhizopogon vinicolor FSL788-5 caused a significant increase in net photosynthesis rate compared to non-mycorrhizal controls, while Hebeloma crustuliniforme HeCr2 and Laccaria laccata S238-A had no effect. Colonization by Rhizopogon and Hebeloma caused increased osmotic potential in the leaf symplast compared to controls, while Laccaria did not. Colonization levels for Rhizopogon, Hebeloma and Laccaria were 36, 93 and 73 % of root tips, respectively. Rhizopogon and Hebeloma produced abundant extramatrical hyphae and/or rhizomorphs, while Laccaria was smooth-mantled. Hebeloma-colonized seedlings were significantly smaller than non-mycorrhizal controls; Rhizopogon seedlings were smaller, but significantly so only at P < 0.10. Laccaria did not affect seedling size. Only smaller Hebeloma seedlings exhibited elevated concentrations of N, P, K, and Ca over non-mycorrhizal controls. These data demonstrate a non-nutritional basis for increased rate of photosynthesis caused by some ectomycorrhizal fungi that can be explained by the increased photosynthate sink generated by extensive fungal growth associated with the mycorrhizas.