A total of 11,598 wild-caught females of Culex vomerifer and 229 specimens of other species of Culex (Melanoconion) mosquitoes captured in Almirante, Panamá, were allowed to feed on 54 laboratory-bred hamsters. Of the C. vomerifer given access to hamsters, 3,987 were observed to engorge on blood. A total of 18 transmissions of arboviruses were demonstrated in hamsters bitten by C. vomerifer mosquitoes, and one strain of Ossa virus was obtained from a hamster fed on by C. taeniopus. Nine of the transmissions by C. vomerifer, five of Ossa virus, and four of Madrid virus, were identified by isolation of the agents from the tissues of hamsters bitten by these mosquitoes. The other nine transmissions, five of Guamá, and one each of Ossa, Madrid, Patois, and Mayaro viruses, were demonstrated and identified by subjecting the serum of surviving hamsters to HI and CF tests against antigens of 27 different arboviruses. All five viruses transmitted were previously known to occur in the Almirante area, and four of them are commonly encountered there. It is concluded that C. vomerifer is a proved important vector of Ossa, Madrid, and Guamá viruses in Almirante, the first two of which are important pathogens to human beings in the area. It, however, does not seem to be an efficient vector of VEE in the study area even though it is frequently found harboring this viral agent. The species C. taeniopus is suspected to be a vector of Ossa and perhaps of other arboviruses when high population peaks are attained.