Abstract Sawada, K., Takahashi, H., Abe, K., Ichii, T., Watanabe, K., and Takao, Y. 2009. Target-strength, length, and tilt-angle measurements of Pacific saury (Cololabis saira) and Japanese anchovy (Engraulis japonicus) using an acoustic-optical system. – ICES Journal of Marine Science, 66: 1212–1218. Pacific saury and Japanese anchovy generally congregate in dense groups or schools. An acoustic-optical system (the Japanese Quantitative Echosounder and Stereo-video Camera System or J-QUEST) has been developed to measure accurately the target strength (TS) of fish in a dense school. J-QUEST comprises a quantitative, 70 kHz, split-beam echosounder and a stereo-video camera. It was deployed from a research vessel to collect concomitant measures of TS and stereo images of in situ Pacific saury and Japanese anchovy. The stereo-video camera provides estimates of the fish lengths (L) and tilt-angles corrected for J-QUEST motion. In this way, empirical models of TS vs. log(L) were derived for Pacific saury and Japanese anchovy and compared with theory.