In working over the Trichosphaeriaceae preparatory to a monograph of the family a number of points of interest have arisen relating both to genera and species. In order to bring out some of these points the North American species of the genus Lasiosphaeria are here described and illustrated. The genus Lasiosphaeria was founded in 1863 by Cesati and De-Notaris, with Sphaeria ovina Pers. as type of the genus. In I869 Fuckel took up the genus Leptospora Rabenh. but used it in a different sense from that in which it was originally used by Rabenhorst, including Sphaeria ovina Pers. in this genus. In the diagnosis of the genus Leptospora Fuckel states: Diese Gattung steht, was den Sporenbau anbelangt, Lasiosphaeria nahe, nur sind bei Leptospora die Sporen ohne Querwanden. In the members of this group of plants it is very difficult to rely upon the septation of the spores as a basis for generic distinction since in many species the spores are nonseptate when young and it is difficult to find mature spores, but when mature spores are found, they are often delicately separate. The type of the genus Lasiosphaeria as usually collected has nonseptate vermiform spores while rarely plants of the same species are found with some of the spores enlarged at one end into an ellipsoid head and becoming one or more septate. Other species of the genus which usually contain nonseptate spores occasionally have the spores septate without enlargement, the number of septa varying with the species. While Fuckel in his diagnosis of the genus Leptospora regards the spores as nonseptate he includes in the genus species in which, as described above, the spores are often septate. The genus Leptospora of Fuckel is therefore regarded as a synonym of Lasiosphaeria, in which genus the presence or absence of septa is a variable character.