Certain wrasse species (Labridae) are used as cleaner fish in salmon farms on the Norwegian coast, reducing salmon louse intensities. The pathogen repertoire of wrasse in Norway is poorly known, and the objective of the present study is to describe a novel intracellular bacterium detected in Norwegian Labrus bergylta. Histological examination of gill tissues from ballan wrasse, L. bergylta, revealed epitheliocysts occurring basally to the secondary lamellae in the interlamellar epithelium. Ultrastructurally, these had bacteria-filled inclusions with thickened membranes and radiating ray-like structures (actinae). 16S rRNA gene sequences from the gill bacteria showed the highest (97.1 %) similarity to Candidatus Similichlamydia latridicola from the gills of the latrid marine fish Latris lineata in Australia and 94.9 % similarity to Candidatus Actinochlamydia clariae, causing epitheliocystis in the freshwater catfish Clarias gariepinus in Uganda. A total of 47 gill samples from L. bergylta from Western Norway were screened by real time RT-PCR with an assay targeting Candidatus Actinochlamydiaceae 16S rRNA. Prevalence was 100 %. We propose the name Candidatus Similichlamydia labri sp. nov. for this new agent producing gill epitheliocysts in L. bergylta.