A summary is given on ecological and hygienic problems of coastal waters, beginning with an outline of the different standpoints in ecology and hygiene. Because of close interrelationships between these fields, the hygienist is introduced to the ecological problems of the Baltic Sea. Hygienic questions have drawn attention because of the so-called Baltic Sea disease (Red tides?--Indication of virus contamination by bacterial indicators of faeces?). On the basis of the results of a two-year authoritative study of the German Baltic Sea coast, a discussion is presented on 1) the relationship between the coli titer and the frequency of salmonella detection, 2) the necessity of routine tests for salmonella, 3) aims of bacteriological cleanliness, 4) the importance of chemical parameters for evaluation, 5) necessary measures for prevention of pollution of the water and beaches of the Baltic Sea. Emphasis is placed on the fact that for ecological reasons the third stage of sewage purification has attained increased significance. The paper refers repeatedly to international agreements (Baltic Sea Convention of 1974, the 1975 WHO report on the recreational quality of beaches and coastal waters, European Community guidelines of 1975) and their relations to the presented findings and considerations.