Abstract The seasonal growth of Salvinia molesta Mitchell was recorded during the period March 1978 to April 1979 in floating quadrats established in Lake Moondarra and a nearby sewage lagoon. Primary-form plants were used as inoculum in the quadrats; leaf numbers, fresh weight and percentage cover changes were recorded. These three parameters were significantly correlated. The highest individual rates of increase recorded in the lagoon were 51.4% per day for percentage cover, 50.1% per day for leaf numbers and 38.3% per day for fresh weight. In the lake, the highest values were 19.0% per day, 25.7% per day and 16.6% per day, respectively. A simple growth model for this species, relating growth rate to water temperature, accounted for 76% of the observed variance in leaf number growth rates in the lagoon. The same model, however, only explained 5% of the variance in the lake, suggesting growth limitations by other factors.