Among the many insects under investigation as potential vectors of the oak wilt fungus in Wisconsin and elsewhere, the species of Pseudopityophthorus have attracted considerable attention. Although recent developments appear to indicate that the long-distance spread of this serious disease occurs primarily with the aid of other insects, our knowledge at present does not warrant the assumption that bark beetles play no part whatever. In the study of P. minutissimus from August, 1952, to the fall of 1953, considerable information concerning its biology in Wisconsin has been brought to light; and regardless of its relationship to the oak wilt problem it seems desirable at this time to make better known this member of the Wisconsin bark beetle fauna.