Mutants of cyanobacteria defective in parts of the CO₂-concentrating mechanism are classified into three types. (i) Mutants defective in inorganic carbon transporters. One of these mutants was constructed by inactivating cmpA encoding 42 kDa protein in the cytoplasmic membrane. (ii) Mutants defective in NAD(P)H dehydrogenase(s). There are five ndhD genes in Synechocystis PCC6803, two of them expressed constitutively and three inducible by low CO₂. Two kinds of NAD(P)H dehydrogenase appear to be involved in energizing and inducing the high affinity inorganic carbon transport system. (iii) Mutants defective in carboxysome with impaired ccm or icfA genes. New type of mutants with impaired cotA (renamed as pxcA) have also been isolated. These mutants did not show light-induced proton extrusion and were unable to grow at acidic pHs. A mutant constructed by inactivating cotA (pxcA) in the wild-type Synechocystis was unable to transport CO₂ at pH 6.5. We concluded that cotA (pxcA) has a role in light-induced proton extrusion that is essential at acidic pHs to extrude protons produced during CO₂ transport.Key words: CO₂-concentrating mechanism (CCM), CO₂ transport, NAD(P)H dehydrogenase, proton extrusion, carboxysome, mutant.