Moss associated cyanobacterial communities (MAC) are considered to be major contributors to N input in numerous high latitude ecosystems. Information about the composition and diversity of MAC and their role in biological N-fixation in sub-arctic ecosystems is currently lacking. In this study we aim to assess diversity, abundance and activity of N-fixing MAC associated with the four common moss species, Racomitrium lanuginosum, Hylocomium splendens, Pleurozium schreberi and Sanionia uncinata in widespread, moss-dominated Icelandic ecosystems. Diversity and abundance have been assessed by phase-contrast and fluorescence microscopy. Cyanobacterial nifH genes have been amplified and sequenced in order to determine the phylogenetic types of N-fixing cyanobacteria involved. According to morphological features the cyanobacterial strains observed in association with the four moss species probably all belong to the order Nostocales. Sequencing and phylogenetic analysis have showed that Nostoc punctiforme has been the most common cyanobacterial species associated with R. lanuginosum and P. schreberi, while it has not yet been possible to identify cyanobacteria associated with H. splendens and S. uncinata. Real-time quantitative PCR (qPCR) will be used to determine the relative abundance of nifH genes and N-fixation activity of MAC will be assessed in parallel by the acetylene reduction assay (ARA) and validated by uptake of labelled ¹⁵N.