Abstract Small beams from the sapwood of loblolly pine (Pinus taeda L.) were subjected to decay by pure cultures of Poria monticola Murr. and Polyporus abietinus (Dicks.) Fries; similar beams from the sapwood of red gum (Liquidambar styraciflua L.) were subjected to decay by pure cultures of Lenzites trabea (Pers.) Fries and Polyporus versicolor (L.) Fries. Some of the beams were removed and broken in a toughness machine at the end of every 2-week period up to a final decay period of 14 weeks. The reduction in toughness due to decay was very rapid with no important difference in the action of the white rot and the brown rot organisms. The brown rot organisms caused noticeable shrinkage on drying whereas the white rot organisms did not.