Synedra planktonica was originally described by Hains and Sebring from a freshwater locality in the south-eastern USA using light and transmission electron microscopy. The authors placed S. planktonica into the genus Synedra Ehrenberg because of its solitary habit and lack of marginal linking spines. Since the original description of S. planktonica, the concepts of Synedra and the related genus Fragilaria Lyngbye have undergone significant change and debate. Today, details of the areolae, apical pore fields, cingulum and rimoportulae, all lacking in the original description of S. planktonica, are now commonly used to distinguish between taxa in Fragilaria, Synedra and related genera. We provide details of these ultrastructural characters for S. planktonica based on specimens collected from the type locality, along with observations of cells from other sites in North America and Europe. Based on these findings, an emended description is presented for S. planktonica and the taxon is transferred to Fragilaria, as F. longifusiformis comb. nov. et nom. nov. According to the International Code of Botanical Nomenclature, the epithet planktonica could not be applied because it was previously used to describe a marine species of Fragilaria. We discuss the relationship of S. planktonica with morphologically similar taxa, including the genus Reimerothrix Prasad.