In attempts at the biological control of Salvinia molesta in Australia, the curculionid Cyrtobagous sp. was released at eight sites and became established at seven of them. At these seven sites, air temperatures ranged from less than 0°C to more than 45°C and the mean concentration of nitrogen in S. molesta ranged from 1·18 to 1·82% of dry weight. Establishment was effected by releasing 200 adults, and the shortest time for extensive damage to be caused to the weed was four months. Cyrtobagous sp. dispersed in undamaged S. molesta at rates of a few metres per month. The pyralid Samea multiplicalis Gn. was released, and became established, at three sites and spread at least 170 km north and south in 20 months. At the sites colonised, air temperatures ranged from less than 0°C to more than 45°C and the mean nitrogen concentration in Salvinia molesta ranged from 1·34 to 3·30% of dry weight. Establishment was effected by releasing 800 larvae, and the shortest time for extensive damage to be caused to S. molesta was three months. Contrasts in the biology and ecology of the two species of insects are discussed briefly as is competition between them for food. In late 1983, Cyrtobagous sp. had achieved control of S. molesta at five sites, while Samea multiplicalis had achieved control at none. It is suggested that S. multiplicalis might perform better in locations which experience maximum temperatures lower than those in the present distribution.