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Very dense populations of nematodes of the genera Plectus and Panagrolaimus were found in moss cushions and in ornithogenic soils in a number of localities on mountain outcrops (nunataks) penetrating the ice sheet and in a coastal oasis in East Antarctica. These high abundances permitted analysis of the structure of populations from various sites. Patterns of distribution and size-class composition in nematode populations from the Basen nunatak, and from scattered samples taken in a larger area including several nunataks and one coastal oasis in Dronning Maud Land, were investigated. It is suggested that the composition of developmental stages and structure of the nematode populations are influenced by environmental conditions such as food supply, temperature and maybe predation or competition from coexisting populations of tardigrades, protozoans and rotifers. The results indicated that the population dynamics were not synchronised among the sites. In several populations the animals appeared well fed with a high frequency of gravid females. In the more extreme localities, in particular, the proportion of adults was very high, and in these sites the recruitment to the populations appeared to be rather low as indicated by the low proportion of juveniles. The structure of the nematode populations seemed little influenced by various combinations of co-occurring tardigrades or rotifers. There were some differences in population structure between nematode populations from different nunataks which could possibly be caused by genetic variability due to long periods of isolation.