The effects of iron and cobalt on methanogenesis from dairy cattle manure were studied. Four-liter digestors with 3-liter working volumes were charged daily with dairy manure (4.5% W/V volatile solids) to achieve a 3-day retention time. Digestors were incubated at 37°C and pH 7.0 on a rotary shaker. Duplicate digestors were maintained under the following parameters: controls (no ions added), Co++ at 4.958 mg/liter, Fe++ at 20.64 mg/liter and Fe++ and Co++ at above stated final digestor concentrations. Significantly higher production of biogas and methane occurred with the addition of iron (p = .05). Iron-amended digestors produced 3.88 ± 0.26 liters/liter/day of biogas and 2.03 ± 0.14 liters/liter/day of methane. Control digestors produced 3.59 ± 0.27 liters/liter/day of biogas and 1.85 ± 0.14. Cobalt did not stimulate methanogenesis and may have nullified the stimulating effects of iron. Neither the cobalt nor the iron/cobalt-amended digestors demonstrated increases in biogas or methane production. Iron or cobalt did not affect efficiency of fermentation (liters of methane per gm of volatile solids destroyed) or volatile fatty acid conversion. The total counts of methanogens grown in roll tubes were unaffected by addition of the metals. Methanobrevibacter smithii was isolated from all digestors, however, Methanobacterium formicicum was isolated only from digestors amended with iron.