Motile thecal cells derived from the hatching of single cysts identified asSpiniferites membranaceusandS. ramosushave been used to establish cultures. These cultures were examined in order to assess the cyst-theca relationships of these two taxa. The cultures produced two different motileGonyaulaxspecies belonging to Kofoid'sSpiniferagroup. These cultures were then induced to form a new cyst generation under uniform conditions, and examination of large numbers of the resulting cysts has shown that process development is an extremely variable phenomenon although process morphologies display a continuum within a species. Process length (and to a certain degree, process morphology) requires careful interpretation when used to discriminateSpiniferitestaxa, in both modern and ancient environments.