The eut operon of Salmonella typhimuriumencodes proteins involved in the cobalamin-dependent degradation of ethanolamine. Previous genetic analysis revealed six eutgenes that are needed for aerobic use of ethanolamine; one (eutR), encodes a positive regulator which mediates induction of the operon by vitamin B₁₂ plus ethanolamine. The DNA sequence of the eut operon included 17 genes, suggesting a more complex pathway than that revealed genetically. We have correlated an open reading frame in the sequence with each of the previously identified genes. Nonpolar insertion and deletion mutations made with the Tn10-derived transposable element T-POP showed that at least 10 of the 11 previously undetected eutgenes have no Eut phenotype under the conditions tested. Of the dispensable eut genes, five encode apparent homologues of proteins that serve (in other organisms) as shell proteins of the carboxysome. This bacterial organelle, found in photosynthetic and sulfur-oxidizing bacteria, may contribute to CO₂ fixation by concentrating CO₂ and excluding oxygen. The presence of these homologues in the eut operon ofSalmonella suggests that CO₂ fixation may be a feature of ethanolamine catabolism in Salmonella.