The introduction and spread of exotic species in various parts of the world is regarded by many as a major threat to global biodiversity, and freshwater fish are both significant vectors of, and vulnerable to, this threat. Many ornamental fish are brought into Australia for the aquarium trade, and between 12 and 14% of Australians are thought to keep aquaria. Thirty species of liberated aquarium fish are known to have established self-sustaining populations in Australia. The genetic threats of these exotic species are likely to be: 1) hybridisation and introgression; 2) problems associated with small populations owing to deleterious ecological interactions and disease; and 3) hybridisation among exotic taxa. Hybridisation, introgression and the breakdown of species boundaries pose a significant risk to biodiversity throughout the world. The old paradigms of biological species being reproductively isolated do not hold under empirical analysis. Particular groups, such as fish and plants, readily hybridise, and hybridisation and introgression indeed appear to be an intrinsic part of the evolutionary process. This presentation will discuss these threats and review existing data on the current situation in Australia.