Caccolithopliorids are important members of phytoplankton communities in most parts of the world's oceans. The formation, release and flux of the calcite scales (coccoliths) of these organisms over millions of years, has led to sedimentary deposits of great geological significance. The reporting of immense blooms of coccolithophorids by satellite photography and their passible involvement in atmospheric sulphur emissions has emphasised the necessity for further information on their distribution and ecology. During an extensive sampling programme in the N.E. Atlantic, water samples were collected throughout the photic zone at 30 stations. Scanning electron microscopy of filtered water samples enabled coccolithophorid family and species distributions to be compiled. It was found that certain species assemblages could be assigned to distinct biogeographic zones and that these zones closely conformed to the movements of the N. Atlantic surface currents. However, at the Azores Front it was shown that a transitional region existed between the cold and warm water masses, where the flora of each zone could be found. Detailed studies of coccolithophorid vertical distribution has shown that most species can be assigned to discrete depth bands within the euphotic zone. Furthermore, it was noticed that in two species which have no depth preference, the degree of calcification increased with depth. One of these species, E. huxleyi, displayed two ecomorphotypic variants differing in their calcification, size and morphology. During two east to west transects it was noticed that the halocaccolitiiophorid numbers increased towards the Mid-Atlantic Ridge. A number of new species are described and current taxonomic problems reviewed.