Certain megaspores from the genusSelaginella(Lycophyta) and similar fossil genera of Cretaceous and Tertiary age are known to demonstrate a remarkable, iridescent appearance. Recent work on the spore wall ultrastructure shows that this iridescence is produced by a complex, ordered, particulate organization which is analogous to that of iridescent virus aggregates, precious opal and other synthetic materials. Our results suggest that this effect is produced by the accumulation of the spore wall material in the form of a monodisperse colloidal crystal. Colloidal organizations can also account for adjacent non-iridescent wall layers, the transition zones between these and the colloidal crystal and for spore walls with no hint of regular organization. The brief time in which the distinctive ordered exine ultrastructure develops and its occurrence around non-viable spore protoplasts add weight to the hypothesis that the exine is largely self-assembling. This new model of spore wall formation may well have wider implications in terms of current concepts of exine organization and development.