Because of their small size and limited mobility aquatic insects are seldom recognized as invasive species and only where verifiable historical data exist can introductions of exotic species be confirmed. Records of some generally considered cosmopolitan species could in fact be undocumented introductions. Of the entirely aquatic orders of insects only the Odonata are noted as regular invaders. Trichoptera, Ephemeroptera, Plecoptera and Megaloptera are all poor colonizers. In contrast, the Hemiptera, Diptera and Coleoptera have many species which show excellent colonizer and invader qualities. Diptera are recognized as the most deleterious of introduced aquatic insects because many of the species are pests and disease vectors of man and his livestock. Although no alien aquatic insect pests have been recorded in South Africa, the potential for the introduction of problem species exists.