The monoraphid genus Cocconeis ranks among the architecturally most complex members of the diatoms because of the heterovalvarity of its raphe- and sternum valves and because of its generally complex girdle bands. A morphologically and ultrastructurally based revision of 11 previously poorly described marine alveolate Cocconeis species and a description of two new species, Cocconeis materoxanae sp. nov., Cocconeis barleyi sp. nov. and a new variety, Cocconeis pellucida var. curvirotonda comb. nov., are presented, based on LM and EM observations. Features common to all these species are their general morphology and the presence of chambered areolae (alveoli) on the sternum valve (SV). which open to the valve interior through a circular foramen. The material examined originated from various herbarium collections as well as from freshly collected epiphytic diatom communities on leaves of the seagrass Posidonia spp. sampled off the coast of Australia and in the Mediterranean Sea. The presence of diagnostic characters common to the alveolate taxa described here and to the other alveolate well-defined species leads us to propose the establishment of the new section ? alveolatae ? within the genus Cocconeis. The relationships between the new alveolatae section and the other sections of the genus are also discussed.