Oral Bifidobacteriaceae, Bifidobacterium dentium and Bifidobacterium longum, are known to be isolated together with mutans streptococci and lactobacilli from caries lesions, suggesting that these Bifidobacteriaceae are caries associated and acid resistant. This study aimed to investigate effects of acidification on B. dentium and B. longum, and to compare them with those on Streptococcus mutans, Streptococcus sanguinis and Lactobacillus paracasei. Effects of acidification, growth ability in a complex medium at a pH of 4.0-8.0, cell viability in 2-morpholinoethanesulfonic acid monohydrate (MES)-KOH buffer at pH 4.0, as well as stability of intracellular pH (pH(in)) at an extracellular pH of 3.5-8.0 estimated using a fluorescent dye, 5(6)-carboxyfluorescein diacetate N-succinimidyl ester in MES-KOH, 3-(N-morpholino)propanesulfonic acid-KOH or N,N-bis(2-hydroxyethyl)glycine-KOH buffer, were investigated. B. longum grew as well as Streptococcus strains over a wide pH range, whereas B. dentium grew best in the narrow pH range around neutral. The cell viability of B. dentium decreased significantly after 2 h of acidification at a pH of 4.0, but this was significantly less than that of the Streptococcus and Lactobacillus species, whereas B. longum maintained almost 100% viability. The pH(in) was close to the extracellular pH at pH of 5.5-7.5 in the Bifidobacterium and Streptococcus strains, while at a pH of <5.0, the pH(in) was higher than the extracellular pH in all the strains, but the pH(in) maintenance ability of Bifidobacterium strains was higher than that of the Streptococcus strains. The high survival rate and pH(in) maintenance ability of bifidobacteria comparable to that of S. mutans in the acidic environment may account for why bifidobacteria exist as stable species in acidic caries lesions together with mutans streptococci.