Achlya ambispora sp. nov. occurring on litter (floating dead leaves and twigs) in Las Cotofla stream, Tierra del Fuego Province, Argentina, is described and illusbated and cornpared with similar species. The species produces principally androgynous and monoclinous, rarely diclinous antheridial branches; oogonial wall and oospores are distinctly yellowish at maturity and differ from other species in having also 1–6(-22) centric-subcentric oospores, in oogonia somethes apiculate, that are frequently intercalary or proliferate in chains.