Enzymatic activity for isomerizing d-glucose to d-fructose was demonstrated in the extracts from the d-xylose-grown cells of heterolactic acid bacteria; the enzyme was called the d-glucose isomerase. d-Xylose was found to be the only active source of carbon among the sugars tested for this enzyme production. Supplements of manganese and cobaltous ions to synthetic or malt extract media caused the remarkable acceleration on this enzyme production. d-Glucose and d-xylose isomerases were purified on a column chromatography of DEAE-Sephadex. Optimum pH for both isomerases was found to be at pH 6.0 to 7.0, but Michaelis constants for two substrates were markedly different: 0.52 M for d-glucose, and 1.3×10-2M for d-xylose. Purified enzyme has not shown any activity for dehydrogenation of d-sorbitol or xylitol, and these sugar alcohols inhibited both isomerase activities competitively. So, it was supposed that the isomerization of d-glucose or d-xylose proceeded directly without participation of their sugar alcohols. Both isomerase activities could not separate from each other, and their activities were inhibited in the presence of two substrates, so it would be probably assumed that the d-glucose isomerase was identical with the d-xylose isomerase.