The genetic diversity of Trichodesmium spp. from natural populations (off Bermuda in the Sargasso Sea and off North Australia in the Arafura and Coral Seas) and of culture isolates from two regions (Sargasso Sea and Indian Ocean) was investigated. Three independent techniques were used, including a DNA fingerprinting method based on a highly iterated palindrome (HIP1), denaturing gradient gel electrophoresis of a hetR fragment, and sequencing of the internal transcribed spacer (ITS) of the 16S-23S rDNA region. Low genetic diversity was observed in natural populations of Trichodesmium spp. from the two hemispheres. Culture isolates of Trichodesmium thiebautii, Trichodesmium hildebrandtii, Trichodesmium tenue, and Katagnymene spiralis displayed remarkable similarity when these techniques were used, suggesting that K. spiralis is very closely related to the genus TRICHODESMIUM: The largest genetic variation was found between Trichodesmium erythraeum and all other species of Trichodesmium, including a species of KATAGNYMENE: Our data obtained with all three techniques suggest that there are two major clades of Trichodesmium spp. The HIP1 fingerprinting and ITS sequence analyses allowed the closely related species to be distinguished. This is the first report of the presence of HIP1 in marine cyanobacteria.