Termite alates presumably do not mate in their natal nest or during a swarming event. Rather, after flight, females and males lose their wings, pair, and select a nest site, with mating typically taking place in the incipient nest. In this study of Reticulitermes flavipes (Kollar) (Isoptera: Rhinotermitidae), histology and other techniques were used to assess reproductive features of female and male imagos, including unflown alates, flown alates, and the primary pair in incipient and well-established (5-yr-old) colonies. The spermathecae of female flown alates were devoid of spermatozoa, as expected, but the lumen contained a mucus-like substance. Histochemical tests revealed that this substance was intensely stained by periodic acid-Schiff, which is diagnostic for mucopolysaccharides. This is the first study to confirm that termites possess a mucopolysaccharide secretion in the spermathecal lumen. The secretion may serve as a sperm energy source as has been suggested for some other insects. Transmission electron microscopy subsequently revealed the presence of a semiopaque secretory material in the spermathecal lumen and associated ductules of flown and unflown alates as well as queens. Unflown female alates showed the least reproductive development in terms of spermathecal diameter and terminal oocyte size. Unflown male alates similarly possessed less developed reproductive organs than the kings.