ABSTRACTPointed, rod-shaped bacteria colonizing the cuticular surface of the hindgut of the terrestrial isopod crustaceanPorcellio scaber(Crustacea: Isopoda) were investigated by comparative 16S rRNA gene sequence analysis and electron microscopy. The results of phylogenetic analysis, and the absence of a cell wall, affiliated these bacteria with the classMollicutes, within which they represent a novel and deeply branched lineage, sharing less than 82.6% sequence similarity to knownMollicutes. The lineage has been positioned as a sister group to the clade comprising theSpiroplasmagroup, theMycoplasma pneumoniaegroup, and theMycoplasma hominisgroup. The specific signature sequence was identified and used as a probe in in situ hybridization, which confirmed that the retrieved sequences originate from the attached rod-shaped bacteria from the hindgut ofP. scaberand made it possible to detect these bacteria in their natural environment. Scanning and transmission electron microscopy revealed a spherically shaped structure at the tapered end of the rod-shaped bacteria, enabling their specific and exclusive attachment to the tip of the cuticular spines on the inner surface of the gut. Specific adaptation to the gut environment, as well as phylogenetic positioning, indicate the long-term association and probable coevolution of the bacteria and the host. Taking into account their pointed, rod-shaped morphology and their phylogenetic position, the name “CandidatusBacilloplasma” has been proposed for this new lineage of bacteria specifically associated with the gut surface ofP. scaber.