In the chemostat, growth of the organisms is limited by maintaining one essential nutrient concentration below optimum. A constant feed of medium, with one nutrient in limiting concentration and with constant removal of culture at the same rate, is used to achieve the steady state. The dilution rate is set at an arbitrary value, and the microbial population is allowed to find its own level. By appropriate setting of the dilution rate, the growth rate may be held at any desired value from slightly below the maximum possible to nearly zero. This constitutes a self-regulating system and allows selection of a desired growth rate. A combined electrolysis-chemostat method, developed by Magna Corp., maintained the hydrogen-producing electrode of an electrolysis cell in the bacterial culture. Resting cells of _Hydrogenomonas eutropha_ consumed hydrogen produced at the cathode of an electrolysis cell built into a specially constructed Warburg flask. Attempts to immobilize _Hydrogenomonas_ cells on a porous conductor were partially successful. This system could lower the volume requirements compared with those for the isolated subsystems. Disadvantages of this integrated system include electrolysis of the bacterial medium, possibly resulting in toxic breakdown products, and the possible effects of electric power and the KOH electrolyte on the bacteria. The main disadvantage of an integrated system would be the disparity between optimal conditions for efficient electrolysis and efficient bacterial conversion, particularly temperature and pH, with the combination possibly resulting in considerably higher power and weight demands. Both continuous-culture approaches are being studied with NASA support. The turbidostat offers the greatest potential efficiency in weight and volume, but uses nutrient materials less efficiently and is more complex. The chemostat is less efficient in weight and volume, but has greater simplicity and reliability. _Hydrogenomonas eutropha_ has been grown in 15-liter batch cultures and in 2.1-liter continuous cultures. A 20-liter continuous culture, sufficient to balance the requirements of a man, is under development. The potential problem areas in large-scale continuous production of the bacteria include assuring genetic stability, preventing or controlling bacteriophage and foreign bacterial contamination, and preventing heterotrophic growth caused by exposure to organic material from the urine. Genetics of hydrogen bacteria and phage infection have been studied by DeCicco. Research on these problems indicates that they are not of major importance, but cause significant effects and must be eliminated or controlled. Bacterial Composition and Nutrition