From June 2000 to September 2001, we investigated the presence of eggs spawned in Margaritifera laevis and the seasonal changes in the gonads of Tanakia tanago. Eggs were observed from mid-March to mid-September. In females with a shrunken ovipositor, as the GSI gradually increased, most ovaries were in the prespawning phase (Oct–Mar). As the GSI increased further, most ovaries were in the early spawning phase (Mar–Jun). As the GSI gradually deceased, ovaries in the late spawning phase appeared (Jun–Sep). When the GSI was very low, most ovaries were in the postspawning phase (Sep–Oct). In males, when the GSI was low, most testes were in the early prespawning phase from Oct–Dec. As the GSI gradually increased, most testes were in the late prespawning phase (Dec–Jan). As the GSI increased further, testes were in the early spawning phase (Jan–Jun). As the GSI gradually decreased, amost testes were in the late spawning phase (Jun–Sep). When the GSI was very low, most testes were in the postspawning phase (Sep–Oct). These results indicate that T. tanago has a distinct annual reproductive cycle and is a spring–autumn spawner. Based on the relationship between reproductive activity and environmental factors, the spawning season of T. tanago appears to be initiated by increasing temperature and / or longer days in spring and to be terminated by shorter days in autumn.