ABSTRACT: Cysts of the dinoflagellate Gymnodinium catenatum were present only in the top sections of duplicate marine sediment cores from Deep Bay in southern Tasmania, Australia. ²¹⁰Pb and ¹³⁷Cs analyses indicate that the appearance ofthe cyst of this toxic dinoflagellate (one of the causative organisms of paralytic shellfish poisoning) occurred after 1972. This sediment core evidence and the absence of this species from the phytoplankton of most other neighbouring Australian waterssuggest that Gymnodinium catenatum is not endemic to Tasmania but has been introduced recently. This species was first seen in bloom proportions in Tasmania in 1980, with major blooms having occurred since then in 1986, 1991 and 1993. Several linesof evidence suggest that ballast water discharge from cargo vessels originating from Japan and South Korea, or less likely Europe, is the most probable mechanism of introduction.