The last decade has seen significant advances in our understanding of thephysiology, ecology, and molecular biology of chemoautotrophic bacteria. Manyecosystems are dependent on CO2 fixation by either free-living orsymbiotic chemoautotrophs. CO2 fixation in the chemoautotroph occursvia the Calvin-Benson-Bassham cycle. The cycle is characterized by three uniqueenzymatic activities: ribulose bisphosphate carboxylase/oxygenase,phosphoribulokinase, and sedoheptulose bisphosphatase. Ribulose bisphosphatecarboxylase/oxygenase is commonly found in the cytoplasm, but a number ofbacteria package much of the enzyme into polyhedral organelles, thecarboxysomes. The carboxysome genes are located adjacent tocbb genes,which are often, but not always, clustered in large operons. The availabilityof carbon and reduced substrates control the expression ofcbb genes inconcert with the LysR-type transcriptional regulator, CbbR. Additionalregulatory proteins may also be involved. All of these, as well as relatedtopics, are discussed in detail in this review.