The cyamids collected from a True's beaked whale (Mesoplodon mirus), a common dolphin (Delphinus delphis), and a harbour porpoise (Phocoena phocoena) stranded in Galicia (NW Spain) were identified as Isocyamus deltobranchium, extending the range of this species to the Atlantic Ocean. The 3 cetacean species examined are new hosts for this parasite. The microscopic examination of female specimens reveals that certain important taxonomic characters may be present or absent depending on the maturation stage, a taxonomic feature that should be considered in future studies describing cyamid species.