Sicyopterus japonicus (Teleostei) is an amphidromous goby that spawns in freshwater, and their newly hatched larvae migrate downstream to the sea where they have an oceanic larval life before migrating back to the rivers to grow and reproduce. The purpose of this study was to understand their recruitment ecology by conducting a field survey in the Ota River, Wakayama, Japan. Otolith Sr: Ca ratio analysis of 15 adults found no evidence of a river resident type of life history. Otolith daily increments of 123 larvae indicated that they spent 173-283 days (mean: 217 days) in the ocean. Their oceanic larval durations were not different among years. To understand their recruitment season, daily set net sampling at the estuary was conducted from March to September in 2006, 2007 and 2008. Though the number of larvae varied among years, 2-6 days of peaks in catches occurred from April to June in the three years. Larvae were collected on days with water temperatures of 15-20 degrees (C), and fewer larvae were caught on rainy days, but there was no relationship with the lunar cycle. Most larvae recruited during daytime, and upstream migration was observed 4-6 hours after low tide. Though the observed recruitment pattern of S. japonicus was apparently not related with daily environmental factors, it was hourly synchronized with the high tide.