The genomic sequence of the archaeon Methanosarcina mazei has been analyzed by the Z curve method. The Z curve is a three-dimensional curve that uniquely represents the given DNA sequence. The three-dimensional Z curve and its x and y components for the genome of M. mazei show a sharp peak and relatively broad peak, respectively. The cdc6 gene is located exactly at the position of the sharp peak. Based on the known behavior of the Z curves for the archaea whose replication origins have been identified, we hypothesize that the replication origin and termination sites correspond to the positions of the sharp peak and broad peak, respectively. We have located an intergenic region that is between the cdc6 gene (MM1314) and the gene for an adjacent protein (MM1315), which shows strong characteristics of the known replication origins. This region is highly rich in AT and contains multiple copies of consecutive repeats. Our results strongly suggest that the single replication origin of M. mazei is situated at the intergenic region between the cdc6 gene and the gene for the adjacent protein, from 1,564,657 to 1,566,241 bp of the genome.