Ambient vibration tests and analyses of a long-span rigid-frame bridge were conducted using distributed long-gauge fiber-optic sensors. The concept of a long-gauge fiber-optic sensor and its merit to reveal local and global structural features are presented. Monitoring of a long-span bridge was performed using long-gauge sensors, and a method to calculate structural deformation distribution from the measured long-gauge strains is proposed, in which shear-deformation effect is considered because the main girder of the studied bridge is deep with a length-depth ratio of 17.9 at the end of the middle span. Modal identification of the studied bridge using the measured long-gauge dynamic strains was also performed, from which strain and displacement mode shapes were identified. Those results demonstrate the superiority of long-gauge fiber-optic sensors and their successful application to the studied bridge. (C) 2016 American Society of Civil Engineers.