The Swelinia Stream is one of several streams discharging into the Gulf of Gdansk in the Sopot coastal zone. The stream is inhabited by a great variety of benthic diatom flora, among which freshwater alkaliphilous diatoms, typical of eutrophic waters, are predominant. The $-meso- and $/'-mesosaprobic diatoms typical of waters moderately polluted by organic matter prevail within the saprobic groups. Xenosaprobic diatoms occur only occasionally. Quantitative analysis of diatoms, classified according to their saprobic requirements into taxa sensitive, tolerant and resistant to organic pollution, has shown that the water quality is deteriorating downstream. Evidence for this was the absence of class I water in the stream's lower course; the usual quality level is class II, which may even drop to class III in the summer and autumn.