A method forin situinoculation of Scots pine roots withFomes annosusis described. Replacement of the fungus within the inoculum and rate of movement of the fungus on to the root surface were two important considerations in the development of a successful inoculation technique. Preliminary observations on host vigor, food-base size, and resin production as factors influencing growth ofF. annosusare noted.In laboratory and field trials, spores ofFomes annosuscaused infection of Scots pine roots only when sown on woody tissues exposed by cutting. A method is described whereby it was possible to observe development of spores on the surface of pine roots. The influence of microbiological and nutritional factors on infection of roots byF. annosusspores is discussed.