Abstract: We have determined the complete 1,694,969 nucleotide sequence of the GC-rich genome of Methanopyrus kandleri using a novel approach. It is based on unlinking genomic DNA with the ThermoFidelase version of M. kandleri topoisomerase V and cycle sequencing directed by 2?-modified oligonucleotides (Fimers). 3.3× sequencing redundancy was sufficient to assemble the genome with <1 error per 40 kb. Using a combination of sequence database searches and coding potential prediction, 1692 protein-coding genes and 39 genes for structural RNAs were identified. M. kandleri proteins show an unusually high content of negatively charged amino acids, which might be an adaptation to its high intracellular salinity. Previous phylogenetic analysis of 16S RNA suggested that M. kandleri belonged to a very deep branch, close to the root of the archaeal tree. However, genome comparisons, using both trees constructed from concatenated alignments of ribosomal proteins and trees based on gene content, indicate that M.