Morphological analysis of Gephyrocapsa spp. in plankton samples confirms the existence of five out of six morphotypes that were previously reported from Holocene sediments. Our data suggest a much higher diversity within the genus Gephyrocapsa than the currently accepted species circumscriptions. Furthermore, we confirm the morphological species delineations made by Kamptner that allow the separation of three morphological groups within the genus Gephyrocapsa: one group with large bridge angles (G. oceanica var. typica Kamptner 1943), a second group with small bridge angles (G. oceanica var. californiensis Kamptner 1956) and a third group of small coccoliths (G. aperta Kamptner 1963). However, a seemingly continuous transition from small to large coccoliths within G. oceanica var. typica along a temperature gradient points either to a high phenotypic plasticity of G. oceanica var. typica or numerous sibling species highly adapted to specific environmental conditions. Testing of these hypotheses is of utmost importance to understanding the diversity of marine plankton and its evolution, and to assessing the impact of future and past environmental change on primary producers such as coccolithophorids. © 2008 Elsevier GmbH. All rights reserved.