The relationship between the pH of the medium and specific growth rates, in well-buffered media at 38.5 degrees C, was determined for three strains of Butyrivibrio fibrisolvens and for one strain each of Streptococcus bovis, Selenomonas ruminantium subsp. lactilytica. Megasphaera elsdenii, Veillonella alcalescens, and Propionibacterium acnes. The pH optima for growth were between 6.1 and 6.6 for all six species, and the upper pH limits were between 7.3 and 7.8. The lower limit pH values for growth on glucose were 5.4 for B. fibrisolvens, near 5.0 for V. alcalescens, and between 4.4 and 4.8 for the other four species. These values fall within the minimum pH ranges found when these species are grown in poorly buffered medium with nonlimiting glucose concentrations. Acid sensitivity per se could cause the washout of B. fibrisolvens, but not of the other five species, from the rumens of animals on high-starch diets.