An ecological study of freshwater waterbodies on Ile de la Possession (Crozet Archipelago, Antarctica) was undertaken during the austral summer of 1997/1998. Both diatom and chemistry data were collected. A highly diverse diatom flora of 210 taxa, belonging to 32 genera, was identified from 127 samples. The most abundant genera were Fragilaria, Achnanthes and Navicula. Principal Component Analysis was used to classify the samples on the basis of their chemical characteristics. The analysis revealed that pH and chloride were the main factors dividing the samples into three categories: rivers, more acid lakes, and lakes with higher chloride content. Species assemblages correspond well with this division. Forward selection with Monte Carlo permutations selected pH, chloride, habitat and hardness. Based on the analysis, four groups of samples can be made: rivers, acid lakes, circumneutral lakes with low chloride concentrations and coastal pools with relatively high chloride levels. The results also indicate that a specific diatom flora exists on the island, which is comparable to that of other islands in the same area (Kerguelen and Macquarie).