The assimilability of glucose and xylose of Rhodopseudomonas K-7, whose hydrogen evolution has been characterized previously, was investigated under the anaerobic photosynthetic and the aerobic dark conditions. This organism is able to grow well in the medium containing glutamate and malate as organic substances under the anaerobic light condition. However, the substitution of glucose for malate retarded the growth rate, while the addition of glucose to the seed culture remarkably promoted the utilization of glucose added in the main culture. Optimal glucose concentration in the seed culture to induce glucose assimilability of the organism was around the concentration of 60 mM of glucose. Then, the seed culture grown in the medium containing 60 mM of glucose were inoculated in the medium containing 10, 20, 30, 60 and 100 mM of glucose respectively. The results were revealed that the consumable content of glucose was limited even though the high concentrations of glucose was contained in the medium. The consumption of considerable amount of glucose was observed when cultured under the aerobic dark conditions than the anaerobic illuminated conditions.