Phosphoglucose isomerase has been purified from crude extracts of Escherichia coli K 10. Two forms of the enzyme were separated during the purification procedure. The major species comprises more than 90% of the enzyme activity, has an apparent molecular weight of about 125,000 and consists of two 59,000 molecular weight subunits; the minor species has an apparent size of 230,000 and consists of (possibly four) subunits of 59,000 molecular weight. Both enzyme forms have the same N-terminal amino acid, the same pH optimum of reaction and the same kinetic constants for the substrate fructose-6-phosphate and the inhibitor 6-phosphogluconate. They differ in that the minor species has half the specific enzyme activity compared to the major one and that its subunit polypeptide carries a higher electronegative charge. Since they are both coded by the pgi gene and since they show full immunological identity it seems that the minor species is a dimer of the major enzyme form and that dimerisation is caused by subunit modification. No physiological role could be found for the existence of the two forms. -- Formation of phosphoglucose isomerase is under respiratory control: under anaerobiosis the enzyme (both species) is depressed parallely with other glycolytic enzymes.