The tropical fresh water fern salvinia(Sa1vinia mlesta D.S. Mitchell) ' is a problematic weed and is being controlled biologically by the introduction of a weevil, Cyrtobagous salviniae. The plant consists of three morphologically different growth forms; primary, secondary and tertiary, duringitsgrowth. The present paper describes how the initial density of primary stage plants affected its morphological plasticity. Primary stage apical cuttings consisting of three ramets (uniform in size) were selected and were grown at densities of 16,400 and 800 ramets per square metre. The growth and morphological changes were observed for a period of 100 d. Plants remained at their primary stage under the lowest density and they transformed from primary to secondary and then to tertiary growth stages at higher densities. There were significant differences between the growth forms and their growth.