An aerobic, pink-pigmented, budding and rosette-forming bacterium was isolated from an acidic Sphagnum peat bog and designated strain A10T. The 16S rRNA gene sequence analysis showed that strain A10T was a member of the order Planctomycetales and belonged to a phylogenetic lineage defined by the genus Gemmata, with 90 % sequence similarity to that of Gemmata obscuriglobus, the only taxonomically described organism of this group. Ellipsoid-shaped cells of strain A10T were uniformly covered with crateriform pits and possessed long (up to 10–15 µm) and unusually thick (0.5–0.7 µm) stalks of a unique ultrastructure. Thin sections revealed a complex intracellular membrane system compartmentalizing the cells. Strain A10T was a moderately acidophilic, mesophilic organism capable of growth at pH values between 3.8 and 7.2 (with an optimum at pH 5.5–6.0) and at temperatures between 10 and 30 °C (with an optimum at 20–25 °C). The major fatty acids were C18 : 0, C18 : 15c and C16 : 15c and the major quinone was MK-6. Cells of strain A10T contained high amounts of bound saturated and monounsaturated C26–C32 (-1) hydroxy fatty acids. The G+C content of the DNA was 62.5 mol%. The unique cell morphology, the capability of growth in acidic conditions and a number of chemotaxonomic and genotypic characteristics served to differentiate strain A10T from G. obscuriglobus. Based on these data, the novel isolate should be considered as representing a novel genus and species of planctomycetes, for which the name Zavarzinella formosa gen. nov., sp. nov. is proposed The type strain is A10T (=DSM 19928T=VKM B-2478T).