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Trichodesmium Ehrenberg species were collected in the Caribbean Sea, Sargasso Sea, and coastal areas of Tanzania (Indian Ocean). The specimens were divided into five species on the basis of morphometric characters such as cell dimensions and colony formation: T. tenue Wille, T. erythraeum Ehrenberg, T. thiebautii Gomont, T. hildebrandtii Gomont, and T. contortum Wille. In addition, Trichodesmium sp., a spherical colony of uncertain taxonomic position was examined. The cell structure of each species was investigated by means of light, scanning electron, and transmission electron microscopy. Particular attention was paid to the presence and ultrastructural arrangement of gas vacuoles and glycogen fiber clusters (GFCs). This resulted in identification of two major groups of species: 1) T. tenue, Trichodesmium sp. with spherical-shaped colonies, and T. erythraeum with GFCs and more or less localized gas vacuoles; and 2) T. thiebautii, T. hildebrandtii, and T. contortum lacking GFCs and with gas vacuoles spread at random. The specie, within each grop were further characterized with respect to the dimesion of the gas veside, cylindrical bodies, scroll bodies, and a new cellular inlusion body. Differnces in colony formation and cell dimesions correlated with specific ultraatructural character in five of the six forms. This is the first ultrstrucrural study comparing different forms of Trichodesmium sampled at grographically remote areas and shows that one species appears identical rrgarless of the sampling site. Some of the species had lint been investigated earlier, and probably more species are to be identified and analyzed. [ABSTRACT FROM AUTHOR]