The effects of environmental temperature on Ockelbo virus infection, dissemination, and transmission were studied in Culex torrentium and in the Uppsala and the El Gabal strains of Cx. pipiens. Temperatures tested included 10°C, 17°C, 24°C, and a cyclic (10–24°C, mean = 17°C) regimen designed to mimic typical hourly temperatures in an area endemic for Ockelbo disease in Sweden during the transmission season. The vector competence of both the Uppsala and the El Gabal strains of Cx. pipiens was directly related to environmental temperature. Mosquitoes held at 10°C had a reduced ability to transmit virus as compared to those held at 17°C, 24°C, or in the cyclic temperature regimen. A rapid increase in dissemination rates was observed in mosquitoes exposed to a shift in temperature from 10°C to 24°C. After 4 days at 24°C, these mosquitoes had vector competence similar to those held at 24°C for the entire incubation time. In contrast, virus dissemination in Cx. torrentium was rapid at all temperatures tested and appeared unaffected by environmental temperature; infection and dissemination rates were consistently higher in Cx. torrentium than in either the Uppsala or the El Gabal strains of Cx. pipiens. It seems the transmission of Ockelbo virus by Cx. pipiens might be interrupted by a prolonged period of cold weather, while transmission by Cx. torrentium would continue.