Preliminary studies on distributions of attached algae in British estuaries suggest that the estuaries might be classified on the basis of the flora in the opper reaches. This could contribute to biological assessment of water quality in estuaries. This paper reports on a preliminary comparison of algal distributions in a larger number of estuaries resulting in the recognition of three algal types of estuary. Some factors which might be involved in the maintenance of one of the types are also discussed. It is possible that one of the factors involved in the dominance of Melosira nummuloides in the Clyde is the restriction of potential grazers which will be due to some other environmental factor(s) such as low dissolved oxygen, low salinity or toxic effluents.