The tropical green seaweed, Caulerpa taxifolia has been invading the northwestern Mediterranean Sea since 1984, and has been discovered recently in California. This strain of C. taxifolia has proved an aggressive colonist, capable of vigorous growth on a variety of substrates. It can displace the native seagrasses and seaweeds, and its progress remains unchecked by grazers, which either avoid it or are poisoned by it. A variety of unsuccessful attempts have been made to curtail its spread. In this report we document the susceptibility of C. taxifolia to ionic aluminium. The exposure of whole fronds to 1 mM AlCl3 causes complete inhibition of photosynthesis in less than 2 hours. The effect is irreversible. The ecologically important seagrass Posidonia oceanica is unaffected by this treatment. The unusually low internal pH of 4.0 found in Caulerpa taxifolia may explain its vulnerability to aluminium poisoning, since the highly toxic Al3+ ion predominates at low pH. The results of this study provide the basis for the development of an environmentally sound means of selectively reducing the dominance of the aggressive strain of C. taxifolia in areas where it adversely affects the native plants and animals.