In this paper we provide a brief summary of recent work that supports the notion that the carboxysome, a polyhedral body containing Rubisco, plays a pivotal role in the cyanobacterial CO2-concentrating mechanism. This work includes (i) experiments in which active human carbonic anhydrase II was expressed in the cytosol of the cyanobacterium Synechococcus PCC7942 resulting in a high CO2-requiring phenotype, and (iii) characterization of two types of high CO2 requiring mutants of Synechococcus that appear to be incapable of generating CO2 within the carboxysomes. Carboxysomes appear to serve as a microcompartment where CO2 can be generated and elevated at the site of carboxylation. We also report on the identification by complementation and sequence analysis of a 300 base pair open reading frame that is located upstream of rbcL and that is involved in the correct functioning of the carboxysome. Preliminary electron microscopy data is also considered on the biogenesis of carboxysomes in Anabaena variabilis M3. Key words: carboxysome, cyanobacteria, carbonic anhydrase, CO2-concentrating mechanism, genetic analysis, mutants.