Tube-dwelling diatoms are found in a variety of habitats, from marine to fresh water on different types of substrata; rock, wood, plant and animal. I have described species as tube-dwelling (Cox, 1975 b) if they produce mucilage which is consolidated into a tubular structure around the cells, yet within the mucilage tube individual cells move and divide. The tube is usually attached to the substratum at its base and is extended apically by the secretion of more mucilage by the enclosed diatoms. Tube volume thus increases to accommodate the increased endotubular diatom population. The presence of tube-dwelling diatoms on rocky shores has been recorded by some workers (Aleem, 1949, 1950; Castenholz, 1963, 1967; Hopkins, 1964), while Castenholz (1963, 1967) correlated seasonal and distribution patterns of some littoral species with their response to light intensity and day-length. Carter (1932,1933) described the algal flora, including several tube-dwelling species, of two salt marshes but little work has been done on tube-dwelling diatoms occurring in estuaries. During the course of a study on the biology of these diatoms (Cox, 1975 ft) variation in their distribution at two Severn Estuary sites prompted an investigation into their distribution in the estuary as a whole. Diatom distribution was recorded with reference to the geographical location in the estuary and to the position on the shore at two sites lower down the estuary (Clevedon and Weston-Super-Mare).