The most distinctive feature of the reproductive biology of Hoplosternum littorale is the building and guarding of an elaborate floating bubble nest. Reproduction starts at the age of one year. Males grow to a larger size than females and, during the breeding season, they develop fat deposits in the pectoral fin and an elongated recurved pectoral spine that often assumes a reddish colour. Analysis of gonadal development and surveys of nesting in swamps show that initiation of reproduction is associated with the onset of the rainy season. Males start building the nest at night, and spawning takes place around noon the day after. Most nests are built in newly flooded swamps, especially in open water in the peripheral area of the swamp. A minimum distance of 10 m between nests is usually observed. The floating bubble nest consists of a dome of plant material supported by oxygen-rich foam produced by the male. Diameter and height of the nest average 30 and 6 cm, respectively. The nest is the centre of a territory that is vigorously defended by the male, using its enlarged pectoral spine, against conspecific and heterospecific intruders. The number of nests that did not acquire spawns (48 %) suggests a bold competition among males for females. There are indications that the female drinks the sperm and that fertilization takes place after the sperm has passed through her digestive tract. Hoplosternum littorale is a multiple spawner. Investment in reproduction is high in females since they can spawn up to 14 times during a 7-month breeding season and each spawn consists of 6 000 to 9 000 eggs. On average, two to four females spawn simultaneously, resulting in an average number of 20 000 eggs per nest. Incubation of the eggs takes two to three days depending on the temperature in the nest. Despite intense predation pressure on eggs and larvae, the guarding behaviour of the male extends for only one or two days after hatching. In the hypoxic water of tropical swamps, the main function of the floating bubble nest appears to be to provide oxygen to the developing eggs by lifting the eggs above the water surface while protecting them from desiccation. Other functions of the nest may be protection of the brood against predators, temperature regulation, identification of the centre of the territory and synchronization of reproductive activities. We suggest that H. littorale may be a useful model for the study of the reproduction of tropical freshwater fish in hypoxic environments.