Culturing experiments of the intermediate morphotype of the cosmopolitan coccolithophore Calcidiscus leptoporus, indicate that the size of its coccosphere and of its coccoliths are affected only in a minor way by temperature. The changes observed in clones growing under different temperature and light conditions are within the range defined for this morphotype in the plankton and Holocene sediments. This outcome suggests that the three morphotypes of living C. leptoporus may be reproductively isolated species rather than stages in a life cycle of a single species or ecophenotypic adaptations of a single species with considerable morphological plasticity. Numerous extinct morphotypes of C. leptoporus have been recorded from marine sediments deposited during the last approximately 25 Ma. In the light of our experiments, these may in fact represent genetically distinct species, which experienced rapid evolution.