As part of a study of the diversity of planctomycetes, two novel strains, designated OJF2(T) and OJF8, were isolated from the sediments of a freshwater aquarium. The organisms were chemoheterotrophic, spherical and pink-pigmented, had an optimum growth temperature of about 30-35 °C and an optimum pH for growth of around 7.5-8.5. The predominant fatty acids were C(18:1)ω9c and C(16:0). The two strains were able to assimilate several sugars and organic acids. 16S rRNA gene sequence analysis confirmed the affiliation of these organisms to the phylum 'Planctomycetes'; they showed highest similarity to the type strains of Singulisphaera acidiphila (92.4%) and Isosphaera pallida (91.9%). On the basis of physiological, biochemical and chemotaxonomic characteristics, strains OJF2(T) and OJF8 are considered to represent a novel species of a new genus of the order Planctomycetales, for which the name Aquisphaera giovannonii gen. nov., sp. nov. is proposed. The type strain of Aquisphaera giovannonii is OJF2(T) (=CECT 7510(T) =DSM 22561(T)).