ABSTRACT A form of Triparma laevis (Parmales, Bolidophyceae), not formally described yet, has been reported in Southern Ocean phytoplankton surveys for more than 30 years. A re-examination of some filtered seawater samples collected off the Antarctic Peninsula in the 1960s by the USNS Eltanin provided an opportunity to make detailed morphological observations. The new taxon, Triparma laevis f. marchantii f. nov., is morphologically very similar to T. laevis f. inornata from the subarctic Pacific, but differs in having a C-shaped ridge (rather than a straight one) in the centre of the shield and ventral plates, and a more rounded Y-shaped ridge on the dorsal plate. The other infraspecific taxa of T. laevis have distinct features lacking in T. laevis f. marchantii. The new taxon was sporadically the most abundant Parmales near the Antarctic Peninsula, but lower in abundance in other areas around Antarctica.