Sixteen samples of two soil cores (about 550 and 180 cm in depth) were drilled at intervals in the lower reach of Heihe river basin (northwest of China) in order to illustrate soil microbial characteristics and diversity of culturable bacteria in an extreme by arid environment. Soil water content, organic matter, total nitrogen, pH, direct cell counts, and culturable microorganism counts were evaluated. The total cell concentration was 19-1120/microg (i.e. 0.19-11.2 x 10(8) per g) soil, the culturable bacteria count being 0.2-10.9 per microg (i.e. 2 x 10(5)-10.9 x 10(6) CFU/g) soil. The number of direct cell counts obtained by 4',6-diamidino-2-phenylindole-staining or the cound of culturable microbes after enrichment with different media were statistically significantly correlated with soil organic matters, total nitrogen content, soil water content and surface vegetation; this partly explained the larger number in the deeper first core than in the shallower one. As part of identification of 228 colonies isolated from the two cores, thirty-two were selected for 16S rDNA amplification, sequencing and molecular identification. These 32 isolates were affiliated to 5 major groups of bacteria: alpha-Proteobacteria, 5-Proteobacteria, gamma-Proteobacteria, the high-G+C G+-bacteria, the low-G+C G- -bacteria, and the Cytophaga-Flexibacter-Bacteroides group. Twenty-eight were rod- or short-rod shaped, which accounted for >87.5% of all species; only 4 of 32 species were cocci (<12.5%).