The floral biology of Nymphaea mexicana Zuccarini has been investigated. Anthesis is diurnal with flowers opening and closing for two consecutive days. Flowers on the first day of anthesis are functionally female with the stigmatic papillae secreting a small quantity of fluid. Total dissolved solids of the stigmatic fluid range from 3 to 4%; glucose and fructose are in equal concentrations and a large number of free amino acids are present. Flowers are primarily visited by Dialictus bees, flies and beetles. These insects typically land on the wet stigmatic surface where the secretion loosens pollen from the insects' body. On the second day of anthesis, the anthers dehisce and the stigmatic fluid disappears. This latter phenomenon is associated with a loss of stigmatic receptivity. Since protogyny is complete, flowers are obligatorily entomophilous. After anthesis, the flower closes and submerges. The perianth and androecium abscise and decompose as the fruit matures. The fruit yields large trichomic seeds. The phylogenetic position of N. mexicana within the genus is discussed.