Mosquitoes that feed on crocodilians are poorly known, despite the potential role of these exothermic animals as reservoirs of arboviruses. In this article, we assessed the frequency, abundance, and temporal variation of caiman-biting mosquitoes as well as searched for the natural vectors of the blood parasite of caimans, Hepatozoon caimani, in the Pantanal area of central-western Brazil from captures conducted bimonthly from September 2006 to September 2007 and in February 2008. A total of 5,272 mosquitoes belonging to 10 species of Þve genera was caught on caimans. The most abundant species were Culex (Melanoconion) theobaldi, Mansonia (Mansonia) titillans, Mansonia (Man.) humeralis, and Mansonia (Man.) amazonensis, which together accounted for 80% of all sampled individuals. Other blood-feeding Melanoconion species were also found quite frequently on caimans, including Culex clarki, Culex idottus, and Culex bastagarius. Oocysts of H. caimani were exclusively detected in Culex species, mainly in individuals of the subgenus Melanoconion, and we accomplished experimental transmission from naturally infected mosquitoes to uninfected Caiman yacare. The highest infection rates were observed in Cx. theobaldi (0.55%), which is therefore indicated as the primary vector of H. caimani. In addition, because the above mentioned Melanoconion and Mansonia species are abundant, widespread, and have a broad set of hosts, including crocodilians, they may be suggested as potential vectors of arboviruses, such as West Nile virus, in the Southern Cone in South America.2030-01-01