Allelopathy refers to non-resource-based interactions between primary producers that may affect the outcome of competition. The ‘Novel Weapon Hypothesis’ predicts that non-indigenous plants have the potential to become invasive if they produce allelopathic substances that the native species are not adapted to. The alien waterweed, Elodea canadensis, has become a noxious weed around Australia. We examined the allelopathic effect of E. canadensis on two species of cyanobacteria, Anabaena variabilis and Synechococcus sp. The growth of the target species exposed to E. canadensis exudates and live material was compared to that of controls (i.e. neither exudates nor live material present). We found significant negative effects on the growth of the two target organisms with the strongest effect exerted on Synechococcus sp. Our findings correspond with the predicted hypothesis and suggest that allelopathy plays an important role in the success of E. canadensis.