Four monoraphid taxa belonging to the genera Achnanthes,Psammothiclium and Planothidium were found during the ongoing taxonomic revision of the freshwater and limno-terrestrial diatoms of the Maritime Antarctic region. The present paper describes these four taxa as new based on detailed light and scanning electron microscopy observations: Achnanthes kohleriana Kopalová, Zidarova & Van de Vijver sp. nov., Planothidium wetzelectorianum Kopalová, Zidarova & Van de Vijver sp. nov., Psammothidium confusoneglectum Kopalova, Zidarova & Van de Vijver sp. nov. and Psammothidium superpapilio Kopalova, Zidarova & Van de Vijver sp. nov.
The morphology and ecology of all four taxa are discussed and the species are compared with morphologically similar taxa.