The waterweed Lagarosiphon major (Hydrocharitaceae) was first recorded in Lake Taupo in 1966, and by 1979 it had occupied most, if not all, potentially colonisable sites in the lake. The invasion pattern of L. major in Lake Taupo showed that the barriers to its spread were almost entirely non- botanical. Correlations of the distribution of this plant with sediment type, water depth, slope, aspect, and exposure to wave action showed that the latter was the most important variable controlling its ultimate distribution. In ideal conditions, L. major forms tall monospecific stands which eliminate the native flora, over a depth range of 1 to 6.5 m. Very few species-rich stands of native aquatic plants now exist in Lake Taupo. Until now the human-related niusance value of L. major has attracted considerable attention, but the implications of the spread of this species for the conservation of the indigenous flora in this and other lakes have been overlooked.