In order to test whether morphological variations within Gephyrocapsa oceanica var. typica Kamptner 1943 reflect genotypic variation or phenotypic responses to environmental conditions, culture experiments of six strains of G. oceanica collected at different locations in the North Atlantic and Mediterranean have been carried out under different temperature and nutrients conditions. All morphological, and physiological data suggest the presence of two species or subspecies within G. oceanica var typica that correspond morphologically to Gephyrocapsa "Larger" and possibly to Gephyrocapsa "Equatorial" as previously defined from Holocene sediments. Given the importance of Gephyrocapsa species for the carbon cycle in the past, genetic studies on this group are of major interest to the understanding of past climate change and plankton evolution. © 2009 Elsevier GmbH. All rights reserved.