A new neogregarine pathogen of the great spruce bark beetle, Dendroctonus micans (Coleoptera: Curculionidae, Scolytinae), is described based on light microscopy and ultrastructural characteristics. The pathogen infects the fat body and the hemolymph of the beetle. The infection was nonsynchronous so that different developmental stages could be observed simultaneously in the hemolymph. All life stages from sporozoite to oocyst of the pathogen including micronuclear and macronuclear merozoites were detected. The sporozoites measured about 8.7 x 1.9 mu m and trophozoites, 11.9 x 3.3 mu m. Micronuclear merozoites seen in the hemolymph were motile, elongate, slightly broader at the anterior pole, and measured 18.4 x 2.0 mu m. Macronuclear merozoites had a size of ca. 16.4 x 2.3 mu m. Gametogamy results in the formation of two paired oocysts within a gametocyst. The lemon-shaped oocyst measured 10.9 x 6.1 mu m and had a very thick wall (375-450 nm). All morphological and ultrastructural characteristics of the life cycle stages indicate that the described neogregarine in D. micans is clearly different from known Mattesia species infecting bark beetles, and from any other described Mattesia spp. Therefore, we create a new species, Mattesia weiseri sp. nov.