Spore forming actinobacteria (sporoactinobacteria) isolated from soils with an acidic pH in Pinus thunbergii forests and coal mine waste were subjected to taxonomic characterization. For the isolation of acidophilic actinobacteria, acidified starch casein agar (pH adjusted to 4-5) was used. The numbers of actinobacteria growing in acidic media were between $3.2{ imes}10^4$ and $8.0{ imes}10^6$ CFU/g soil. Forty three acidophilic actinobacterial strains were isolated and their 16S rDNA sequences were determined. The isolates were divided into eight distinctive phylogenetic clusters within the variation encompassed by the family Streptomycetaceae. Four clusters among them were assigned to the genus Streptacidiphilus, whereas the remaining four were assigned to Streptomyces. The clusters belonging to either Streptomyces or Streptacidiphilus did not form a monophyletic clade. The growth pH profiles indicated that the representative isolates grew best between pH 5 and 6. It is evident from this study that acidity has played a critical role in the differentiation of the family Streptomycetaceae, and also that different mechanisms might have resulted in the evolution of two groups, Streptacidiphilus (strict acidophiles) and neutrotolerant acidophilic Streptomyces. The effect of geographic separation was clearly seen among the Streptacidiphilus isolates, which may be a key factor in speciation of the genus.