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Perianth differentiation into distinct morphological whorls (sepals and petals) can be difficult to assess, particularly in early-divergent angiosperms. The perianth of members of Nymphaeales (Cabomba, Brasenia, Barclaya, Euryale, Nuphar, Nymphaea, Victoria, Ondinea) has been described as differentiated into sepals and petals, undifferentiated or presenting examples of both states. In this paper, we review perianth structure in Nymphaeales using morphology and ontogeny in order to determine whether their perianths are dimorphic. The results indicate that although there are differences between the outer and inner perianth series, the organs display few "typical" sepal-petal characteristics. Our observations of the perianth of Nuphar and Nymphaea, and reports on Euryale, show that sepaloid (green) and petaloid (yellow) areas can both occur on individual perianth organs, and that sepaloid areas occur mostly in the regions of the perianth organ that were exposed when the flower was in bud. SEM study revealed further morphological differences between the sepaloid and petaloid areas of Nuphar and Nymphaea and also between the exposed and covered regions of the perianth of Cabomba, Brasenia and Barclaya. We discuss the implications of these results with respect to current concepts of perianth differentiation.