Two Basidiomycetes, Fomes pini (Fr.) Karst. and Peniophora pseudo-pini Weres. & Gibson, and three microfungi (Ascomycetes), Tympanis hypopodia Nyl., Retinocyclus abietis (Crouan) Groves & Wells, and a member of the Coryne sarcoides complex, are the fungi most frequently isolated from the heartwood of living jack pine (Pinus banksiana Lamb.) in the Province of Ontario. Of these, only F. pini is consistently associated with decayed wood. In laboratory tests all five caused significant weight losses in jack pine heartwood samples, and all but the C. sarcoides complex appreciably altered the color of the wood. T. hypopodia was exceedingly antagonistic towards F. pini, both in malt agar culture and when inoculated in advance of F. pini in jack pine wood samples. The activity of P. pseudo-pini in the test samples was reduced slightly by the prior establishment of T. hypopodia and the C. sarcoides complex, but was stimulated somewhat by the presence of R. abietis. The presence of the C. sarcoides complex in jack pine samples resulted in a moderate increase in the activity of F. pini.In some cases competitive interactions among heartwood-inhabiting fungi may partly explain the marked variability in defectiveness frequently observed among individual trees in forest stands.