Diatoms, dinoflagellates and coccolithophorids are the most important groups in the marine phytoplankton community. Studies of coccolithophorid taxonomy and ecology in the Adriatic are scarce, unlike those on diatoms and dinoflagellates. Although 86 species of coccolithophorids were reported for the northern Adriatic, only 11 species are regularly recorded. A revision of coccolithophorid biodiversity and their ecological importance in the Adriatic was necessary due to new insights on the life cycles of species. This study was conducted during one year (2008 2009) and samples were collected every ten days in the coastal area in front of Rovinj. The research was completed using light and electron microscopic examinations together with pigment analysis by high-performance liquid chromatography. A total of 52 coccolithophore morphospecies were recorded during the study. Dominating genus was Syracosphaera, represented by 13 species. Coccolithophorids of both diploid (HET) and haploid (HOL) phases were present throughout the investigated period. Seasonality in HET/HOL phases was observed for Syracosphaera pulchra (summer)/ HOL Calyptrosphaera oblonga (spring) and Coronosphaera mediterranea (autum)/ HOL Calyptrolithina wettsteini (spring). Pigment analysis pointed to a clear domination of 19-hexanoyloxyfucoxanthin, the pigment characteristic for haptophytes. High concentrations of 19-hexanoyloxyfucoxanthin were recorded throughout the year with a distinct peak in the spring. The spring period (March - June) was characterised by a high species diversity and higher contribution of coccolithophorids to the total phytoplankton assemblage (>60%). The typical summer coccolithophores, Rhabdosphaera clavigera and Syracosphaera pulchra, were related to high temperatures and low nitrate concentrations. Coccolithophorids Calciosolenia murrayi and C. brasiliensis were related to elevated nutrient concentrations, and were characteristic for the autumn assemblage. They were able to find favourable living conditions despite the abundant occurrence of diatoms and low light transmission values. Highest abundances of the species Emiliania huxleyi (106 cells L-1 ) were recorded in January during a monospecific bloom (92% of whole phytoplankton community). E. huxleyi was the most frequently recognised species and was related to low temperature and nutrient rich waters, consequently present in high numbers both during winter and summer.