A survey is given of the epilithic algal vegetation occurring in the tidal area of the SW Netherlands during the period 1966-1973, in relation to important environmental factors. The area has been divided into seven sections, from the North Sea coast up to and including the freshwater tidal areas. In each section several algal communities have been distinguished. The distribution patterns of these communities (comprising ca. 200 species of green, brown, red and blue-green algae spread over 49 communities) have been correlated with macrogradients in the environment: salinity of the water, exposure to wave action and the amount of sediment deposited on the dike-slopes.