Rumen microorganisms were used in a batch fermentor to investigate whether they could be used to produce acetic acid, propionic acid, butyric acid, and methane from cotton fibers. The kinetics of product formation were studied with the initial medium pH values of 6.0, 7.0, and 8.0. Propionic acid, butyric acid, and methane production were growth associated; acetic acid production was found to be nongrowth associated. The largest amounts of bacteria and protozoa were attained with the initial pH of 7.0. The highest specific production rate constants were observed for acetic acid with initial pH of 6.0. Almost the same values were observed for the rate constants of propionic acid production with initial pH values of 6.0 and 7.0. No propionic acid and methane were produced at pH 8.0. Butyric acid was produced only when the initial pH was 7.0. The rate constants of methane production decreased with the increasing initial pH values.