Nitrogen is an essential element for life. As part of its biogeochemical cycle, biological nitrogen fixation is a principal means by which inert atmospheric nitrogen is transformed into other metabolizable forms. Its bio-availability, as ammonia or nitrate, could be a limiting factor for the growth and productivity in soil and freshwater microbial systems in Antarctica. In such systems cyanobacteria could be the primary nitrogen fixing organisms. We have analysed some microbial communities near the Uruguayan Antarctic Base Artigas, on King George Island (62°12'S, 58°10'W), in the Maritime Antarctica. During Austral summer large areas of the island become ice-free, with only infrequent patches of microbial mats, lichens and bryophytes. These mats have already been characterized by morphological criteria and Cyanobacteria, including nitrogen-fixing forms, represented the predominant genera. Total DNA from individual communities was isolated, and nifH and 16S rRNA PCR products were amplified for subsequent DNA sequence and phylogenetic analysis. Amplification products exhibited the expected size from most of the analyzed communities, suggesting the presence of cyanobacteria and, in particular, nitrogen-fixing cyanobacteria. Two clone libraries containing approximately 80 clones, corresponding to the 16S rRNA gene were constructed from two different communities. According with the morphological characterization, it was established that one of the communities was dominated by Phormidium and the other by Nostoc. Sequences present in 50 clones from one of the libraries were analyzed by digestion with three different restriction enzymes. Restriction analysis with most of the clones were in accordance with the in silico expected profile for the Phormidium sp. 16S rRNA sequences already known. This information will be used for DNA sequence and phylogenetic analysis as well as to estimate community evenness and complexity. Financed by CSIC, I+D (2004) and Instituto Antártico Uruguayo.