Since 1984, when Alcyonidium gelatinosum (Linnaeus, 1761) was shown not to be the free-growing species hitherto known by that name, it was assumed to be the valid name for A. polyoum (Hassall, 1841). We have established that its neotype is not a specimen of A. polyoum but of A. reticulum Ryland and Porter (2000) introduced for A. mytili auctt. non Dalyell (1848). A. reticulum must accordingly be relegated to the synonymy of A. gelatinosum. A. polyoum is a valid but different species and the provenance of Hassall's specimens is discussed. We deliberate the identity of the nominate species in Linnaeus' Fauna svecica (1761). While the occurrence of A. polyoum in the Kattegat-Mecklenburg Bight sea area cannot wholly be ruled out, the common species is A. reticulum (i.e. A. gelatinosum ). The neotype does therefore represent the Swedish specimen(s) used by Linnaeus, and the identity of A. gelatinosum is at last correctly established.