Arginine biosynthesis in Escherichia coli is negatively regulated by a hexameric repressor protein, encoded by the gene argR and the corepressor arginine. By hydroxylamine mutagenesis two types of argR mutants were isolated and mapped. The first type is transdominant. In heterodiploids, these mutant polypeptides reduce the activity of the wild-type repressor, presumably by forming heteropolymers. Four mutant repressor proteins were purified. Two of these map in the N-terminal half of the protein. Gel retardation experiments showed that they bind poorly to DNA, but they could be precipitated by L-arginine at the same concentration as the wild-type repressor. The other two mutant repressors map in the C-terminal half of the protein. They are poorly precipitated by L-arginine and they bind poorly to DNA. In addition, one of these mutants appears to exist as a dimer. The second type of argR mutant repressor consists of super-repressors. Such mutants behave as arginine auxotrophs as a result of hyper-repression of arginine biosynthetic enzymes. They map at many locations throughout the argR gene. Three arginine super-repressor proteins were purified. In comparison with the wild-type repressor, two of them were shown to have a higher DNA-binding affinity in the absence of bound arginine, while the third was shown to have a higher DNA-binding affinity when bound to arginine.