A new ascothoracid, Isidascus bassindalei gen.nov., sp.nov., has been discovered in the north-east Atlantic Ocean at a depth of over 2000 m. It forms permanent galls on the branches of Acanella arbuscula, a gorgonian of the Family Isidae (Octocorallia, Coelenterata). The new genus is related to Gorgonolaureus, species of which form galls on paramuriceid gorgonians. The gall of the present form unlike that of Gorgonolaureus incorporates a cup of host calcite. Twenty-three adult female specimens and one male were examined in this study. The adult parasite is characterized by elevated humps in a median dorsal position on each of thoracomeres 2,3 and 4. A further feature is a tapering filamentary appendage arising near the base of first thoracopod. The parasite apparently feeds on its host's coenosarc. The likely function of the first and second antennae and each of the three pairs of stylet-form mouth parts is suggested. A case is made for interpretting the species as a protandrous sequential hermaphrodite. Whilst still immature, protanders apparently seek out and settle alongside an adult female. Later, following growth and sexual maturation they again become mobile and after copulation with the nearby female depart to prepare for sex change on a separate branch of the host.