Bull. Jpn. Soc. Fish. Oceanogr. 72(4), Page 273-281, 2008   Maturity and Spawning season of marbled sole Pleuronectes yokohamae, and comparison of reproductive characters between different survey periods in Harima Nada and Osaka Bay, the Seto Inland Sea, Japan Minoru Tanda†, Yukinobu Nakamura1 and Shigeyoshi Okamoto1 1 Fisheries Technology Institute, Hyogo Prefectural Technology Center for Agriculture, Forestry and Fisheries, 22-2 Minami-Futami, Akashi, Hyogo 674-0093, Japan † minoru_tanda@pref.hyogo.lg.jp The spawning season and maturity of the marbled sole Pleuronectes yokohamae were examined from 1986 to 2001 in the waters from Harima Nada to Osaka Bay eastern part of the Seto Inland Sea, Japan. From the seasonal changes of gonadosomatic index (GSI) it was considered that the spawning season was from mid-December to mid-February and the peak period was from late-December to late-January. The age and size at maturity were determined by the GSI in December and January. Maturity rates of 1 year old female fish of 1 year 11 months to 2 year 2 months were 82.1% in 1986-1989 and 99.5% in 1998-2001 (1986-1989 vs. 1998-2001, Fisher’s exact probability test p<0.0001), and those of male fish were 94.1% and 99.6%, respectively (p=0.0003). Approximately 100% of 2 year old male and female fish were mature in both periods. The maturity rates of 1 year old females of 200-219 mm TL were 72.3% in 1986-1989 and 96.6% in 1998–2001 (p=0.004), and those of males of 180-199 mm TL were 90.5% and 100%, respectively (p=0.042). It was suggested that the age and size at maturity of marbled sole have decreased in recent years in this study area. The cause of these changes may be a density-dependent mechanism because of the remarkable decrease of catches in Harima Nada and Osaka Bay from 1986-1989 period to 1998-2001 period. Key words: