various aspects of the biology of chondrostereum purpureum (pers.:fr.) pouz. and its role in dieback and decline of white birch (betula papyrifera marsh.) were studied. (1) sixty-nine trees which exhibited symptoms of dieback and decline were sampled for c. purpureum infection in thunder bay, ontario. using a variety of sampling methods, including boring, and trunk and branch dissection, c, purpureum was isolated from 26 (38%) of 69 trees sampled. the increment borer sampling technique proved to be the most efficient method of isolating the pathogen, in terms of combining ease of execution and good success rate. (2) the pathogenicity of c. purpureum to birch seedlings was tested. all twenty—six seedlings infected with mycelium developed open wounds, 12 of which cankered, whereas the wounds in 26 controls closed within 4—6 weeks. progress of the infection varied. spread of the canker from the wound site (including stem girdling and dieback in two seedlings), spread through the xylem and formation of cankers further up the stem, and apparent containment of the infection at the wound site were observed. (3) the properties of white needle-like crystals which formed on heavy spore casts were investigated. the filamentous crystals were composed of a sesquiterpene compound (cj^gh2go, m^ 222, m.p. 136°c) similar to ( + )-torreyol . (4) the"^sexual ity of c. purpureum was investigated. the fungus is heterothal1ic, with tetrapolar sexual differentiation. anomalies such as unilateral compatibility and unequal abundance of clamps and fruiting in compatible pairings were observed. (5) a method of inducing growth of c. purpureum basidiocarps and their storage was devised.