The type material of Rhizosolenia setigera was examined and compared to isolates identified from Thai coastal waters and culture collections. The type slide, which is mounted in Canada Balsam, contains only a few cells that could be identified as R. setigera. Of those with resolvable girdle bands, multiple segmented girdle bands could be faintly seen as illustrated in the type description of R. setigera. Sequences from isolates identified as R. setigera reveal that it is not a monophyletic taxon. They fall into at least four different molecular clades. Only CCMP 1820, sequenced by Medlin, has a segmented band morphology corresponding to the band morphology of the cells on the type slide. All others have half bands in two to four columns and thus belong to different species. CCMP 1820 has been sequenced by three different labs. That from the Theriot lab is very different (14%) from the other two, which are nearly identical, and it must belong to a different taxon. A new genus Sundstroemia is described to accommodate the major setigera clade and the sequences of CCMP 1820 are assigned to the epithet setigera, whereas its closest sister taxa with half bands in two – four columns are assigned to a new species, S. pauciseries sp. nov. Rhizosolenia pungens and R. similoides are also moved into this new genus, but multiple sequences for these two taxa are not identical and none of the isolates have been compared to the type material of the two species.