A new species of cheilostome bryozoan, Fenestrulina commensalis n. sp., was collected in December 2008 by scuba at 5-10 meters depth at Guaibura Beach, Guarapari, Esp rito Santo state, southeastern Brazil. The specimen was found associated with tubes of the cerianthid Pachycerianthus sp., representing the first commensal association between a bryozoan and a tube-dwelling anemone. Fenestrulina commensalis n. sp. is the third species of the genus found in Brazilian waters; it is distinguished from other Atlantic species of Fenestrulina by its small angular orificial condyles, a single oral spine and basal anchoring rhizoids arising from abfrontal pore chambers. Morphological adaptations to encrust the tubes of cerianthids include anchoring rootlets and weakly contiguous zooids. These morphological features allow the colony the flexibility to grow around the tube and feed relatively undisturbed by silt and detritus, being raised well above the softsediment substratum in which the tube-anemone grows.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Univ Sao Paulo, Ctr Biol Marinha, BR-11600000 Sao Sebastiao, SP, BrazilUniv Estadual Paulista, Fac Ciencias & Letras Assis, Dept Ciencias Biol, Lab Biol Aquat LABIA, BR-19806900 Assis, SP, BrazilUniv Estadual Paulista, Fac Ciencias & Letras Assis, Dept Ciencias Biol, Lab Biol Aquat LABIA, BR-19806900 Assis, SP, BrazilFAPESP: 12/24285-1FAPESP: 12/01771CNPq: 481549/2102-9