The killer algae destroy locally aboriginal florae by their extreme reproductive potential and strong growth power. These green algae, which had been discovered only 20 years ago, are currently causing serious problems in various seas over the world including the destruction of ecosystem of fauna and flora, declines in biomass of useful organisms such as Mollusca and sea urchin, resulting in decline of those catches. Many countries have already taken various actions towards the eradication of killer algae by prohibiting the sale of the algae for ornamental purpose, their cultivation and import for similar purposes. However, there has been no effective law established in Japan so far to regulate increase or import killer algae. It is strongly recommended, under these circumstances, that the measures against killer algae be immediately established and enforced before they would invade and settled down in Japan.