The ecology of coccolithophores (including species’ absolute abundance) in the water column (0-200 m) was studied in the Indian sector of the Southern Ocean, along the 45 degrees E meridian south of Madagascar to Antarctica, during austral summer of 2004 (January–March). Temperature, salinity and nutrient concentrations were measured to establish the relations between environmental parameters and the abundance and species composition of coccolithophore assemblages. Three assemblages were identified: subtropical, subantarctic and polar. The boundaries between the assemblages correspond to the position of the Subtropical Front (STF), the Subantarctic Front (SAF) and the Polar Front (PF). Nutrient concentrations increased from the STF to the PF, and silicate concentrations are negatively correlated to both the abundance and diversity of coccolithophore populations. In contrast, high nitrate concentrations correspond with a high abundance of monospecific assemblages of Emiliania huxleyi at high latitudes. Poleward of the SAF, two morphotypes of E. huxleyi dominate (Type B/C and D).