We examined the distribution, resources and environmental conditions of the clonal aquatic fern Salvinia natans (L.) All., expansive in the Vistula delta (N Poland). Before 2006, there were 7 stands of this species, while in the years 2006-2010 their number increased to 21. The most abundant populations were found in the rivers: Tuga (133.0±37.6 indiv./0.1 m²), Fiszewka (79.3±6.0), Szkarpawa (74.7±5.0), Struga Orłowska (61.0±2.0), Nogat (52.3±2.5), Elbląg (40.3±31.8), Wiślano-Zalewowy Canal (61.3±3.2) and in the SW part of Lake Druzno (72.3±2.5). S. natans did not colonise the weakly saline Vistula Lagoon and Elbląg Bay, which belong to the Baltic Sea. The plant under study occurred in shallow (2.2±1.5 m), narrow (17.9±13.6 m), slow-flowing (0.11±0.12 m s^-1) and fertile (4.7±4.2 mg TN dm^-3, 0.7±0.4 mg TP dm^-3) watercourses. The water in them had neutral or alkaline pH (7.2-9.2) and was weakly saline (53.8±21.3 mg Cl dm^-3). A dense mat of S. natans significantly affected the environmental conditions in the watercourses: water oxygenation, PAR intensity and concentration of biogenic substances, especially phosphorus, decreased.