‘Candidatus Rhabdochlamydia porcellionis' comprises intracellular bacteria within membrane-bound vacuoles in the cytoplasm of hepatopancreatic cells of the terrestrial isopod Porcellio scaber. Cells are non-motile, non-cultivable on cell-free media and have the Gram-negative microscopic appearance of a cell wall without a discernible peptidoglycan layer. Morphology of ‘Candidatus Rhabdochlamydia porcellionis' corresponds to the description of ‘Chlamydia isopodii’ (Shay et al., 1985) and a ‘Rickettsiella-like organism’ (Drobne et al., 1999). Bacteria exhibit a chlamydia-like developmental cycle, in which the cells appear in three morphological forms (reticulate, intermediate and mature elementary bodies) that are packed in membrane-bound vacuoles in the cytoplasm of the host cell. Reticulate bodies with coccoid morphology can be small (up to 1 μm in diameter), which multiply by binary fission, or larger (1–4 μm in diameter), with granular inclusions in the cytoplasm. Morphology of intermediate bodies is coccoid (350–650 nm in diameter), with an electron-dense area in the centre of the cell. Mature elementary bodies have a five-layered cell wall, variable rod-shaped morphology (250–700 nm in length and 100–150 nm in diameter) and oblong structures in the cytoplasm. On the basis of its 16S rRNA gene sequence, ‘Candidatus Rhabdochlamydia porcellionis' forms a distinctive lineage within the order Chlamydiales, close to the family Simkaniaceae, with an oligonucleotide that is complementary to a unique region of the 16S rRNA gene (5′-GAAATGCAAAGGACAGCATC-3′). Assignment to ‘Candidatus Rhabdochlamydia porcellionis' is based on the sequence of the 16S rRNA gene (GenBank accession no. AY223862) and the distinctive morphology of the elementary bodies.