This report shows that isochore-like structures can be found not only in warm-blooded animals, some reptiles, fishes and yeast, but also in certain archaeal species. In perfectly shaped isochore-like structures (in "protoisochores") from Sulfolobus acidocaldarius and Thermofilum pendens genomes the difference in 3GC levels between genes from different "protoisochores" is about 30%. In these archaeal species GC-poor "protoisochores" are situated near the origin of replication, while GC-rich "protoisochores" are situated near the terminus of replication. There is a strong linear dependence between position of a gene and its 3GC level in S. acidocaldarius (an average difference in 3GC per 100,000 base pairs is equal to 3.6%). Detailed analyses of nucleotide usage biases in genes from leading and lagging strands led us to the suggestion that 3GC in genes situated near terminus of replication grows due to higher rates of thymine oxidation producing T to C transitions in lagging strands.