The hydrothermal-vent barnacle Vulcanolepas osheai of the subfamily Neolepadinae is one of the most conspicuous organisms at the Brothers Caldera, south Kermadec Arc, New Zealand. Like a neolepad species found in the Lau Basin, V. osheai harbours filamentous bacteria on its elongated cirral setae. To define the phylogenetic affiliation of the epibiotic bacteria and the nutrition of the barnacle host, we conducted molecular phylogenetic and isotopic analyses. Analysis of 16S rRNA gene sequences of microbial communities on the cirral setae showed that among 91 bacterial sequences investigated, 28 sequences were related to the ɛ-proteobacterial endosymbiont of Alviniconcha aff. hessleri; 11 sequences were related to the epibiont of the bresiliid shrimp Rimicaris exoculata. Fluorescence in situ hybridization showed that in contrary to results from the 16S rRNA gene-sequence library, approximately 80% of the filamentous bacteria hybridized with a probe targeting the sequences related to the epibiont of the bresiliid shrimp R. exoculata. The fatty-acid profiles of the filamentous bacteria and the host barnacle both contained high levels of monounsaturated C16 and C18 fatty acids, and the carbon isotopic compositions of the biomass and monounsaturated C16 and C18 fatty acids of both the bacteria and barnacle were nearly identical. This would suggest that the nutrition of the barnacle is highly dependent on bacteria thriving around the barnacle, including the epibiotic bacteria.