A vulnerability assessment completed in February, 1994 as part of the California Department of Transportation's statewide seismic retrofit program concluded the Carquinez Bridges would require retrofit to meet current seismic resistance standards. These important structures carry Interstate 80 traffic over the Carquinez Strait and serve as the major transportation link between San Francisco and Sacramento. The full retrofit project involves retrofitting the two main spans crossing the Carquinez Strait as well as six approach structures located south of the main spans which constitute the Crockett Interchange. Original construction of these structures was completed in two phases. The first phase, completed in 1927, consists of the main spans currently carrying westbound traffic together with a viaduct approach located at the south end of these spans. All other structures were completed during 1958. This paper describes seismic retrofit of the 1927 main spans.