Abstract Several coccolithophore species are known to exhibit heteromorphiclife cycles. In certain species, notably Emiliania huxleyi, the heterococcolithbearingphase alternates with a non-calcifying stage, whereas in others theheterococcolith-bearing phase alternates with a holococcolith-bearing phase.Heterococcolithophore-holococcolithophore life cycles have previously been observedfor only one species in culture, but have also been inferred from an increasingnumber of observations of combination coccospheres. 18S rDNA sequences frompure cultures of both the heterococcolith-bearing and holococcolith-bearing phasesof Coccolithus pelagicus were identical, providing an additional indication of theiridentity as different life cycle stages of the same species. Flow cytometric analyseshave been undertaken on SybrGreen-stained nuclei isolated from pure cultures ofthe two phases of four coccolithophore species (Coccolithus pelagicus, Calcidiscusleptoporus, Coronosphaera mediterranea and Emiliania huxleyi) in order to determinerelative DNA content. Results confirm the hypothesis that holococcolithophoreheterococcolithophorelife cycles are haplo-diploid in nature. Light microscope observationsof the processes of sexual fusion and meiosis are reported for two of theexperimental species. The results are discussed in the context of the evolution of biomineralizationin the coccolithophores and the possible ubiquity of haplo-diploidyin the haptophytes.