To examine the potential use of synthesis gas as a carbon and energy source in fermentation processes, Phodospirillum rubrum was cultured on synthesis gas generated from discarded seed corn. The growth rates, growth and poly-beta-hydroxyalkanoates (PHA) yields, and CO oxidation/H-2 evolution rates were evaluated in comparison to the rates observed with an artificial synthesis gas mixture. Depending on the gas conditioning system used, synthesis gas either stimulated or inhibited CO-oxidation rates compared to the observations with the artifical synthesis gas mixture. Inhibitory and stimulatory compounds in synthesis gas ccould be removed by the addition of activated charcoal, char-tar, or char-ash filters )char, tar, and ash are gasification residues). In batch fermentations, approximately 1.4 mol CO was oxidized per day per g cell protein with the production of 0.75 mol H-2 and 340 mg PHA per day per g cell protein. The PHA produced from R. rubrum grown on synthesis gas was composed of 86% beta-hydroxybutyrate and 14% beta-hydroxyvalerate. Mass transfer of CO into the liquid phase was determined as the rate-limiting step in the fermentation.