By using a new screening method, we have obtained 43 new Escherichia coli mutants defective in ring formation on galactose swarm plates, which score for defects in chemotaxis or transport. They were complemented and compared with mutations previously known to lie in the galactose-binding protein or the beta-methylgalactoside (mgl) permease, or both. The mutations were all found to lie in three genes, called mglA, mglB, and mglC. mglB codes for the gene specifying the binding protein. Based on co-transduction experiments, mglA, mglB, and mglC lie close to each other on the bacterial chromosome.