Siamese Fighting Fish Betta splendens is a well-known and popular aquarium fish. The species has recently invaded the Adelaide River floodplain, near Darwin in northern Australia. A very large and extensive population has persisted over consecutive years, with potential for further spread. This represents the first serious alien fish species incursion in the Northern Territory, with reports previously limited mainly to livebearers and cichlids in isolated habitats, some of which have been controlled by specific actions reviewed herein. The invasion highlights the need to maintain and strengthen proactive management, community engagement, and coordinated strategies for vectors of introduction including aquarium fishes.