A new species of bacteria oxidizing ammonium with nitrite under anoxic conditions was isolated from the activated sludge of a semi-industrial bioreactor treating digested sludge of the Kuryanovo wastewater treatment plant (Moscow, Russia). Physiological, morphological, and molecular genetic characterization of the isolate was carried out. The cells were ovoid (-0.5 x 0.8 μm), with the intracellular membrane structures characteristic of anammox bacteria (anammoxosome and paryphoplasm); unlike other anammox bacteria, it possessed extensive intracellular membrane structures located in layers parallel to the cytoplasmic membrane, but never close to the anammoxosome. The cells formed aggregates 5-28 μm in diameter and readily attached to solid surfaces. The cells were morphologically labile, easily plasmolyzed, and lost their content. Doubling time was 28 days, μ(max) = 0.025 day(-1); optimal temperature and pH for growth were 20-45 degrees C and 8.0, respectively. Phylogenetic analysis of the 16S rRNA gene sequences suggested its classification as a new species of the candidate genus Jettenia (order Planctomycetales). The name Candidatus "Jettenia moscovienalis" sp. nov. was proposed for the new bacterium.