The vascular anatomy of the f tower is described for nine members of the family Potamogetonaceae Potamogeton indicus, P. natans, P. pectinatus, P. epihydrus, P. praelongus, P. berchtoldi, Ruppia maritima, and Zannichellia palustria. While there are numerous bisexual flowers in a spike of Potamogeton, there are only two in Ruppia. The unisexual flowers of Zannichellia are described as axillary, but vascular anatomy shows that they terminate the axes. The male flower of Zannichellia is regarded here as consisting of one or two stamens. The "flowers" of Polamogeton and Ruppia, which have been considered to be reduced inflorescences, are regarded here as normal flowers. The "perianth" is considered to be true perianth and neither bracts nor expansions from stamen connectives. The triangular outgrowth between the two lobes of the stamens in Ruppia is homologous with the perianth segment of Potamogeton. The vascular supply of a carpel in Potamogeton or Ruppia consists of two bundles[long dash]one dorsal and one placental strand. While the former extends to the base of the stigma in Potamogeton, it is only about half as long as in Ruppia. The only bundle in the carpel of Zannichellia supplies the solitary ovule. The basal part of this bundle in interpreted as representing the fusion product of a dorsal and ventral strand, but higher up, where it becomes inversely oriented, it obviously consists of only the ventral strand. The present study does not give any support to Hutchinson''s placing Ruppia and Zannichellia in separate families.