Floral development in the species Potamogeton zosteriformis is compared with that of other species of Potamogeton previously investigated (P. densus and P. richardsonii). A spike inflorescence is found on which 6 to 20 flowers develop. These flowers develop their appendages acropetally; first, the four tepals are initiated followed by the four stamens opposite the tepals. As in the flower of P. densus, the two lateral stamens are each initiated as two separate primordia. Unlike any other Potamogeton species where four carpels generally arise, alternating with the tepal and stamen pairs, in P. zosteriformis a single carpel consistently develops on the central portion of the remaining floral meristem. Histological preparations did not reveal any vestigial procambial strands differentiating in the gynoecium. This unique floral development is examined from several points of view. Morphogenetic comparisons are made with other species of Potamogeton which develop tetramerous gynoecia. Morphologically the nature of the Potamogeton flower is reexamined and phylogenetically the possibility of P. zosteriformis as a link or transition between the bisexual and unisexual groups within the Najadales is considered.