The structure of the wall at various stages in the life cycle of three species of Thraustochytrium. marine coccoid fungi. has been examined. The cell walls are composed of two elements; thin organic Golgi·derived scales and a fibrillar inter-scale matrix. Cytochemical procedures have established the presence of protein and sulphated polysaccharide in the walls together with associated calcium. The presence of sulphur and calcium has been confirmed by energy dispersive E.M. X-ray analysis. These new features have been considered from the taxonomic viewpoint and appear to strengthen the concept that some affinities exist between the Thraustochytriales and the algal protists.