A comprehensive survey of soil acidobacterial diversity using pyrosequencing and clone library analyses was conducted to characterize the relative abundance, diversity, and composition of acidobacterial communities across various soil types. The study analyzed 87 soils, generating 39,707 and 1,787 acidobacterial DNA sequences using pyrosequencing and clone libraries, respectively. The results showed that Acidobacteria, a widespread and abundant phylum in soil, are strongly influenced by soil pH, with higher abundances in low pH soils. Acidobacteria subgroup 1 was particularly abundant, accounting for 7.4% of bacterial sequences and 17.6% of acidobacterial sequences on average. The abundance of Acidobacteria relative to other bacterial taxa varied across soils but was strongly correlated with soil pH. Soil pH was also the best predictor of acidobacterial community composition, with more phylogenetically clustered communities as pH deviated from neutrality. The study highlights the importance of pH as an effective habitat filter for acidobacterial communities. The findings suggest that pH plays a significant role in regulating acidobacterial abundance and community composition in soil, and that Acidobacteria are often slow-growing oligotrophs. The study also identifies a specific acidobacterial phylotype (phylotype no. 32) that is particularly abundant in soil and may play an important role in soil biogeochemistry. The results emphasize the value of using broad-scale biogeographic surveys to gain insights into the natural history of bacterial lineages represented by few, if any, cultured isolates.A comprehensive survey of soil acidobacterial diversity using pyrosequencing and clone library analyses was conducted to characterize the relative abundance, diversity, and composition of acidobacterial communities across various soil types. The study analyzed 87 soils, generating 39,707 and 1,787 acidobacterial DNA sequences using pyrosequencing and clone libraries, respectively. The results showed that Acidobacteria, a widespread and abundant phylum in soil, are strongly influenced by soil pH, with higher abundances in low pH soils. Acidobacteria subgroup 1 was particularly abundant, accounting for 7.4% of bacterial sequences and 17.6% of acidobacterial sequences on average. The abundance of Acidobacteria relative to other bacterial taxa varied across soils but was strongly correlated with soil pH. Soil pH was also the best predictor of acidobacterial community composition, with more phylogenetically clustered communities as pH deviated from neutrality. The study highlights the importance of pH as an effective habitat filter for acidobacterial communities. The findings suggest that pH plays a significant role in regulating acidobacterial abundance and community composition in soil, and that Acidobacteria are often slow-growing oligotrophs. The study also identifies a specific acidobacterial phylotype (phylotype no. 32) that is particularly abundant in soil and may play an important role in soil biogeochemistry. The results emphasize the value of using broad-scale biogeographic surveys to gain insights into the natural history of bacterial lineages represented by few, if any, cultured isolates.