Glucose 6-phosphate isomerase was extracted from cells of Actinomyces viscosus strain RF-7. The enzyme, with a pH optimum of about 7.6 in HEPES buffer, exhibited Michaelis-Menten kinetics and the apparent Km values for glucose 6-phosphate and fructose 6-phosplate were 4.88 mM and 0.28 mM, respectively. Erythrose 4-phosphate was a strong inhibitor of the enzyme reaction, and 6-phosphogluconate and ribulose 5-phosphate were also competitive with fructose 6-phosphate. The apparent Ki values were 2 μM for erythrose 4-phosphate, 0.11 mM for 6-phosphogluconate and 0.56 mM for ribulose 5-phosphate; the other phosphorylated intermediates were without effect. The enzyme had a molecular weight of 115, 000 as estimated by gel filtration chromatography on Sephadex G-150.