Decay results in appreciable losses in sound wood volume in sugar maple (Acer saccharum, Marsh.) timber in the Ottawa–Huron and Algoma Extension Forest region of Ontario. Seventy-four per cent of 606 sample trees had some measure of decay. Twenty-eight fungi were found to be associated with decay in living trees. The most important of these areArmillaria mellea(Vahl ex Fr.) Quél.,Polyporus glomeratusPeck,Ustulina vulgarisTub,Fomes connatus(Weinm.) Gill.,Hydnum septentrionaleFr.,Corticium vellereumEll. & Crag.,Pholiota spectabilisFr., andFomes igniarius(L. ex. Fr.) Gill. Several species previously unreported in the literature were found to be associated with decay in sugar maple in Ontario.Armillaria melleais the most serious butt-decay fungus andP.glomeratusis the most important trunk-rotting species. Frost cracks are the most important courts of entry for decay fungi. Various discolorations occur in sugar maple. The average extent of decay in living trees is estimated according to the occurrence of sporophores ofF.connatus,F.igniarius,H.septentrionale, andU.vulgaris. The incidence and volume of decay increased progressively with age and diameter. One hundred and nine species of Basidiomycetes are tabulated as occurring on living and dead sugar maple in Ontario.