A new species of ammotheid pycnogonid (Ascorhynchus endoparasiticus sp. nov.) has been found parasitic inside the pallial cavity of an opisthobranchiate mollusc ( Scaphander punctostriatus) from the Azores. One adult and three young individuals were living in three different molluscs, probably feeding upon the rectum contents of their hosts. Many features of the parasites' morphology show various adaptations to this curious habitat.