Abstract The toxicity of SeO 3 −− to wild type Escherichia coli is enhanced by the presence of l-methionine in the growth medium. Data are presented which show that the same degree of toxicity is exhibited by the methionine-requiring mutant of E. coli, A.T.C.C. 9663. d-Methionine is shown to be ineffective. Exogenous methionine suppresses the biosynthesis of selenomethionine as well as methionine. The suppression of selenomethionine biosynthesis provides a basis for the enhanced toxicity of SeO 3 −−. It is also possible that the biosynthesis of selenocyst(e) ine may account for part of the enhanced toxicity. Interaction between SeO 3 −− and methionine as a basis for enhanced toxicity is unlikely from the data obtained on the methionine-requiring mutant, A.T.C.C. 9663.