During a survey of freshwater diatoms from the sub-Antarctic Macquarie Island, a new gomphonemoid diatom was observed. Detailed morphological analysis using both Light and Scanning Electron Microscopy led to the description of Gomphonema isabellae Van de Vijver sp. nov. The results show that this new species belongs to the group of species around Gomphonema acuminatum s.s. Ehrenberg. Preliminary notes on its ecological preferences and the biogeography of the entire G. acuminatum group are presented.