The epiphytic diatoms of filamentous matforming algae in Sombre Lake, Signy Island, South Orkneys, Antarctica were examined in a field study from January to March 1987. Results reveal 1) a shallow shelf assemblage, dominated by Fragilaria spp, with high percentages of empty frustules, and low numbers of viable cells and, 2) a mid-depth zone containing the most luxuriant epiphytic growth dominated by Achnanthes minutisima Kütz., Synedra rumpens var. familiaris (Kütz) Hust., and Cymbella minuta Hilse ex Rabenh., and 3) a deep water assemblage with low numbers of actively growing cells, dominated by Stauroneis anceps var. hyalina M. Perag. et Brun. in Hérib. The comparison of methods and their impact on ecological interpretation is discussed together with a consideration of the principal factors potentially influencing community structure.