Cysts and free spores of a metchnikovellid microsporidium were found in several specimens of an archigregarine Selenidium sp. isolated from polychaetes Pygospio elegans. Samples were collected at the littoral area of the Kandalaksha Bay of the White Sea in the year 2016. We examined this material with high-quality light optics in stained and live preparations. The structure of cysts and the host range suggest that this species belongs to the genus Metchnikovella Caullery et Mesnil, 1897. The length of the cysts varied from 9.5 to 34 .m (av. 23.8 .m); the width of the cysts was 4.8–9.2 µm (av. 8.2 µm). The number of cyst-bound spores varied from 7 to 18. Cyst-bound spores were oval or ovoid and arranged in two or three rows. The length of spores was 2.2–3.0 µm (av. 2.6 µm); the width was 1.4–2.9 µm (av. 1.7 µm). Free spores were similar to cyst-bound ones in shape and size. We summarized available data on the species of the genus Metchnikovella. The analysis of these data proved that the shape and size of the examined cysts and the host range of this parasite differ from those of any known species. We named the observed organism Metchnikovella dogieli n. sp.