Studies of sexuality in Fomes pinicola (Sw.) Cooke based on material from 43 new sources, combined with results already published (1929) have led to the following conclusions: (i) Collections from North America may be divided into two groups, a large Group A, and a small Group B. Monosporous mycelia of any culture in Group A are compatible (mutually fertile) with those of every other member of the group; similarly monosporous mycelia of any member of Group B are compatible with other monosporous mycelia of Group B; but monosporous mycelia of Group A are almost completely incompatible (sterile) when paired with those of Group B. (ii) Collections of European and Japanese origin form a third Group C. These are almost completely compatible with Group A and only partially incompatible with Group B. (iii) In Group B are several isolates of the so-called Populus or hardwood form of F. pinicola which has been designated at times as a separate species F. marginatus. The remainder are, however, the typical coniferous or "red-belt" form.