The genomic sequence of the halophilic archaeon Halobacterium NRC-1 has been 
analyzed by the Z curve method. The Z curve is a three-dimensional curve that 
uniquely represents a given DNA sequence. Based on the known behaviors of the Z 
curves for the archaea whose replication origins have been identified, the 
analysis of the Z curve for the genome of Halobacterium NRC-1 strongly suggests 
that the large genome has two replication origins, oriC1 (921,863-922,014) and 
oriC2 (1,806,444-1,807,229), which are located at two sharp peaks of the Z 
curve. These two regions are next to the cdc6 genes and contain multiple copies 
of stretches of G and C, i.e., ggggtgggg and ccccacccc, which may also be 
regarded as direct and inverted repeats. Based on the above analysis, a model of 
replication of Halobacterium NRC-1 with two replication origins and two termini 
has been proposed. The experimental confirmation of this model would constitute 
the first example of multiple replication origins of archaea, which will finally 
provide much insight into the understanding of replication mechanisms of 
eukaryotic organisms, including human. In addition, the potential multiple 
replication origins of the archaeon Sulfolobus solfataricus are suggested by the 
analysis based on the Z curve method.