The species diversity of the larger intertidal algae was determined at three sites along the coastline of Sydney, New South Wales. The changes in species diversity and species composition were correlated with height from mean of low water (MLW), distance from the edge of the rock platform at MLW, and distance from a sewer outfall (i.e. the degree of pollution). The total number of algal species was reduced in the vicinity of the outfall. This reduction was most evident in the Phaeophyceae and the Rhodophyceae. The maximum value of algal species diversity was also reduced at higher levels above MLW, away from the edge of the platform and near the outfalls.