All members of the genera Haloarcula and Halomicrobium whose names have been validly published were surveyed for 16S rRNA gene polymorphism, and the transcription of the genes from two species was investigated during growth at different NaCl concentrations. The species of Haloarcula and Halomicrobium harbour at least two different 16S rRNA gene copies, and 18 new sequences of 16S rRNA genes were obtained. The type I and type II 16S rRNA genes of Haloarcula are divergent at 4.8-5.6% of their nucleotide positions. The type III and type IV 16S rRNA genes from Halomicrobium mukohataei JCM 9738(T) are 9.0% divergent, which represents the highest intraspecific divergent 16S rRNA genes so far seen. Phylogenetic analysis based on 16S rRNA genes indicated that all type I 16S rRNA genes were clustered, and the same was true for the type II 16S rRNA genes of Haloarcula species. The two clusters, respectively generated from type I and type II 16S rRNA genes, were sharply separated and their divergences (4.8-5.6%) are in the range of various divergence usually found between genera in the order Halobacteriales (about 5-10%). Results from reverse transcription-PCR showed that the type I and type II copies of Har. amylolytica BD-3(T) and type III and type IV copies of Hmc. mukohataei JCM 9738(T) were all transcribed to 16S rRNA molecules under different salt concentrations (15-28% NaCl).