Spawning behavior of the landlocked ayu Plecoglossus altivelis altivelis was observed at five spawning grounds in Lake Ikeda. The spawning season of the ayu in the lake was from early October to late November, when the water temperature falled 18-23°C. Ayu spawned mainly at shore line shallower than about 30cm in depth and the substrate of redds were consisted of fine gravel (φ2-150mm). The number of spawned eggs, wind velocity and direction, and wave size at each spawning ground were measured daily from late September to early November. In most cases, spawning behavior appeared to commence with crosswinds. Spawning behavior was most frequently observed at a wind velocity of 4-5m/sec. These results suggest that crosswinds result in waves which trigger the onset of spawning behavior in landlocked ayu in Lake Ikeda.