5-Keto-4-deoxyuronate isomerase from Escherichia coli has been crystallized after partial purification. The isomerase was found to be enriched in preparations of an unrelated recombinant protein. Crystals of the isomerase were obtained from two different precipitants despite the fact that the recombinant protein represented roughly 90% of the total protein present. The crystals diffract to 2.7 A resolution and are suitable for a structure determination. The role of the isomerase in E. coli is uncertain, as E. coli is not known to degrade the polysaccharides which are potential sources of 5-keto-4-deoxyuronate.