One facultatively and three strictly anaerobic rumen bacteria were grown in pH-controlled anaerobic batch cultures in which the E h of the medium was regulated by the addition of titanium (III) citrate solution at values below −50 mV and potassium ferricyanide above −50 mV. Growth occurred over a wide range of E h, with the maximum limit being +360, + 250, +175 and +414 mV for Selenomonas ruminantium, Bacteroides amylophilus, Bacteroides succinogenes and the aerotolerant Streptococcus bovis, respectively. Changes in E h had little influence on the growth yield or ratios of fermentation end-products in these bacteria over a fairly wide range, although the specific growth rate of all species tended to decline at E h values above 0 mV. Abnormal, elongated forms of Sel. ruminantium and B. succinogenes predominated at high E h. It was concluded that O2, and not a high E h, is the toxic factor in “oxidized” anaerobic growth medium, and that it would not normally be necessary to regulate E h closely in experiments where the growth and metabolism of these bacteria is under study, provided that O2-free conditions were maintained.