Ophiostoma breviusculum sp. nov. (Ophiostomatales, Ascomycota) is a new species in the Ophiostoma piceae complex associated with bark beetles infesting larch in Japan.

A new species of the Ophiostoma piceae-complex was isolated from bark beetles and the insect galleries of Larix kaempferi in Japan. This species was described as Ophiostoma breviusculum. The fungus was morphologically similar to O. piceae and O. quercus. However the average length of the perithecial necks and synnemata were shorter than for O. piceae and O. quercus. Synnemata morphological characteristics also differentiated O. breviusculum from the other species of the Ophiostoma piceae-complex isolated from conifers. Mating tests demonstrated that this fungus did not produce perithecia with O. floccosum, O. piceae and O. quercus. In phylogenetic trees using rDNA ITS O. breviusculum was placed in a clade with O. canum, O. piceae and O. subalpinum, but when using beta-tubulin it was placed into a separate clade.