We have previously demonstrated the involvement of carbonic anhydrase (CA) in larval mosquito midgut physiology. In this study, we used Hansson's histochemistry to examine the distribution of the enzyme in the midgut of Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, Culex nigripalpus, Ochlerotatus taeniorhynchus, Anopheles albimanus and Anopheles quadrimaculatus adult mosquitoes. Additionally, we quantitated CA content in the anterior and posterior midgut of adult males and females from these species using the 18O isotope exchange method coupled to mass spectrometry. We also tested the effect of CA inhibitors such as methazolamide and acetazolamide in the alkalization of the midgut in females from these species. Our results indicate that CA is present in the midgut of adults from the species studied and that it appears to be preferentially associated with the posterior midgut in both males and females. CA inhibitors appear to have a profound effect on midgut pH indicating that this enzyme might play a key role in the maintenance of this pH.