Four day-neutral strains of Scytosiphon lomentaria (Lyngbye) Link were collected along the salinity gradient from the North Sea (32‰ salinity) to the Baltic around Bornholm (8‰) and grown under long-day conditions at temperatures from 5 to 30°C combined with salinities of 30, 20, 10 and 5‰. Results indicate the existence of salinity-temperature ecotypes of Scytosiphon. The inner Danish waters serve as an interface between the seaweed populations of the North Sea and the Baltic.