DON JUAN POND, which contains saturated calcium chloride brine, is in the south fork of the dry Wright Valley of Antarctica at latitude 77°33'S and longitude 167°10'E, and has been controversial almost since its discovery in 1961. Meyer et al.1,2 reported a sparse microflora of four species of heterotrophic bacteria and a yeast. Cameron, Horowitz and colleagues3,4, using the Antarctic dry valleys as the best available natural simulation of Mars, reported that many areas were virtually sterile and, at most, limited to sparse bacterial populations. Field work5-10 has revealed a more abundant and varied microflora of yeasts, blue-green algae, fungi and bacteria, especially in the bottoms of frozen freshwater lakes. However, reports point to an extreme dry valley-exposed rock South-Polar biome consisting predominantly of heterotrophic forms, mainly prokaryotic with occasional fungal associates. If this is correct Don Juan Pond must, like the dry valleys generally, consist only of converter-consumer populations lacking extensive capability for continuous carbon reduction. Cameron has emphasised the ecological restrictions on the activity and distribution of algae in the dry valleys and mentions no algal or other autotrophic forms in his discussion of Don Juan Pond, even though thin organic layers were found 2 m below the 15-20 cm of standing water11. However, in the course of a mercury sampling programme during the austral summer of 1978-79 we observed an extensive, irregular pellicle or mat-like structure 2-5 mm thick but extending 500-600 m2 over much of the western part of the Don Juan Pond salt flats.