the objectives of this study were to investigate the host specificity of mycorrhizal fungi to tamarack (larix laricina (du roi) k. koch.) in pure culture synthesis and to examine the effects of seed sources and fungal species on the ectomycorrhizal formation and growth of containerized tamarack seedlings in a greenhouse. of nine mycorrhizal fungi tested in pure culture synthesis, cenococcum qeophilum, laccaria laccata, laccaria proxima, hebeloma crustuliniforme and pisolithus tinctorius demonstrated their ability to develop ectomycorrhizae with tamarack. amanita porphyria, rhizopogon vinicolor, suillus qranulatus and s. tomentosus failed to form ectomycorrhizae. the number of confirmed mycorrhizal symbionts of tamarack has been increased from three, previously known, to seven from the results of the pure culture synthesis. containerized seedlings of tamarack, representing four provenances and 17 open-pollinated families in ontario, were inoculated with vegetative inocula of four fungal species in the greenhouse. during an 18-week period, l. laccata, p. tinctorius, and c. geophilum formed ectomycorrhizae with 60, 12 and 7% of the total feeder roots, respectively. suillus qranulatus failed to produce any mycorrhizae. the mycorrhizal formation was strongly governed by the seed sources and seed source x fungus interactions. the greatest difference in mycorrhizal formation by l. laccata was 20% between provenances and 32% between families. seedlings inoculated with l. laccata exhibited the best growth in diameter, shoot volume and dry weight. the developrrient of root systems was differentially affected by different fungal inoculations. feeder root proliferation was stimulated by c. geophilum, and lateral root growth was inhibited by l. laccata. significant provenance effects were also found in shoot height and root descriptive variables. although the effects of family-within-provenance were significant in most of the seedling traits, family variation was generally not constant, varying with fungal inoculations. genetic correlations were positive between lateral root and shoot traits and between feeder root frequency and mycorrhizal formation but negative between lateral root and mycorrhizal formation. it is recommended that l. laccata is a suitable fungal species and c. geophilum, h. crustulinlforme, l. proxima and p. tinctorius are potential fungi for mycorrhizal inoculation of containerized seedlings of tamarack. this study reinforces the concept that seed sources should be tested with target fungi before a wide scale mycorrhizal inoculation is conducted.