Four novel strains of budding bacteria, designated MOB10T, PO2, MPL1015 and BG32, were isolated from acidic wetlands of northern Russia. Cells of these four strains were aerobic, non-motile spheres that occurred singly or in shapeless aggregates and attached to surfaces by means of a holdfast material. The isolates were moderately acidophilic, mesophilic organisms capable of growth between pH 4.2 and 7.5 (optimum growth at pH 5.0–6.2) and at temperatures between 4 and 33 °C (optimum growth at 20–26 °C). The strains possessed a complex intracellular membrane system that compartmentalized the cells. The major fatty acids were C16 : 0, C18 : 19c and C18 : 26c,12c. The major quinone was menaquinone-6 (MK-6). The G+C content of the DNA was 57.8–59.9 mol%. 16S rRNA gene sequence analysis showed that strains MOB10T, PO2, MPL1015 and BG32 were members of the order Planctomycetales and belonged to a phylogenetic lineage defined by the genus Isosphaera, exhibiting 90 % sequence similarity to the type strain of the thermophilic planctomycete Isosphaera pallida and 95–95.5 % sequence similarity to a taxonomically uncharacterized group of filamentous bacteria from activated sludge, ‘Nostocoida limicola’ III. However, compared with ‘Nostocoida limicola’ III and Isosphaera pallida, the new isolates from acidic wetlands were non-filamentous, unpigmented bacteria, which possessed highly distinctive phospholipid fatty acid profiles and were capable of growth and of degrading several biopolymers under acidic, microaerobic and cold conditions. The data suggest that the four isolates should be considered as representing a novel species of a new genus of the order Planctomycetales, for which the name Singulisphaera acidiphila gen. nov., sp. nov. is proposed. The type strain of Singulisphaera acidiphila is MOB10T (=ATCC BAA-1392T =VKM B-2454T =DSM 18658T).