©The Author 2017. Published by Oxford University Press. All rights reserved. Coccolithophores are unicellular pelagic algae, capable of calcification. In the Mediterranean Sea, several species have a well-known haplo-diploid life cycle, alternating the production of different types of calcite plates, the holoand hetero-coccoliths. We analyzed the distribution of both phases along a W-E Mediterranean transect during April 2011 and May 2013 (spring season), following strong environmental gradients in salinity, oxygen and nutrient concentration, temperature, carbonate chemistry and fluorescence. The proportion of holococcolithophores:heterococcolithophores of selected species varies not only vertically through the water column, but also longitudinally, following the main environmental gradients. Based on the environmental affinities of the coccolithophore life phases, we conclude that a dimorphic life cycle might provide the ability to adapt to the south-eastern (SE) Mediterranean environment, in conditions characterized by surface water with relatively high calcite saturation state, high temperature, stratification and nutrient limitation, and support the survival of species whose diploid phases are in contrast adapted to Atlantic or south-western (SW) Mediterranean conditions. Thus, a haplo-diploid life cycle could provide a way to adapt to environmental changes.