Cyanobacteria are phototrophic bacteria that play an important role in the terrestrial and freshwater ecosystems of Polar Regions where they accumulate conspicuous biomasses. In addiction, they can be considered as “biosensors” of environment perturbations, for example, the warming of permanently cold environments or the introduction of non-native species. However, the diversity of polar cyanobacteria has not been well explored until now. Previous results have shown that in each new Antarctic sample studied several new genotypes (based on 16S rRNA gene sequences) were found. Among these, many genotypes were recorded in only one sample, and some seemed to have a biogeographic distribution limited to the areas in which they were found. At present the question of the existence of a microbial biogeography is discussed. Some authors propose that local geographical populations of microorganisms exist like is the case of the biogeography of macroorganisms. Other authors propose that microorganisms have such enormous dispersal capabilities due to their small size and large populations , that only different environmental conditions maintain distinctive microbial populations. In order to deepen the knowledge of their diversity and biogeography, we propose to study similar biotopes from the Arctic (Svalbard) and from Antarctica (James Ross Island (Western Antarctica)) and Dronning Maud Land ( Eastern Antarctica )using a polyphasic approach. It will include a molecular study using 16S rRNA gene sequences (DGGE and clone library, aw well as strains), an isolation of cyanobacteria in culture and a microscopic observation of environmental samples and isolated cultures. The samplings and ecological studies as well as the determination of the cyanobacterial morphotype diversity will be done in collaboration with Prof Komarek and Prof Elster ( Czech Republic ).