The breeding ecology of bluegill, Lepomis macrochirus, an invasive alien species in Japan, was studied in the north basin of Lake Biwa in 2002 and 2003. In both years, breeding colonies became established when the water temperature reached at 20°C. In 2002 and 2003, 380 and 192 nests in 55 and 32 colonies, respectively were found at 7 spawning sites, colonies occurring at 1-3 m depth along the ... [Show full abstract] shore. Although the colony size became smaller as the season advanced, large-scale colonies comprising 30 or more nests were found in June (early part of the breeding season). The period of parental care became shorter as the water temperature increased. Breeding success was significantly higher in June than in July and August, also being slightly higher in nests belonging to larger colonies. Predation of eggs in nests by other bluegill, Rhinogobius spp. and Tridentiger brevispinis, was observed, possibly being one of the most important mortality factors of early stage bluegill in Lake Biwa.