Chlorate inhibited the aerobic growth of Escherichia coli K12 in at least two ways, here termed type I and type II inhibition. Type I inhibition was reversed by shikimic acid or by a group of four amino acids (tyrosine, phenylalanine, tryptophane, and methionine), or by nitrate at much higher concentrations. Type I inhibition involved the metabolism of chlorate via nitrate reductase to a toxic form which apparently interfered with aromatic amino acid biosynthesis. The occurrence of type I inhibition is affected by the particular carbon and energy source supplied. Type II inhibition involved chlorate itself, or a derivative not formed by nitrate reductase. The extent of the inhibition was affected by L-serine, methionine, and sodium formate.