Siamese fighting fish Betta splendens in Thailand inhabit shallow water amongst dense emergent vegetation near the margin of rice paddy fields. Nesting water was low in dissolved oxygen, pH, salinity but high in free carbon dioxide and temperature. This fish aggregated with a mean population density of 1·7 fish m 2 and an equal adult sex ratio. Males were heavier, longer and wider than females. The size of their bubble nest is in proportion to their weight and length. 2001 The Fisheries Society of the British Isles