The Haptophyta comprises a group of microalgae of particular importance in marine habitats, often occurring in ‘bloom’ concentrations, sometimes with devastating effects where the bloom is composed of species toxic to other forms of life. The most familiar species are the coccolithophorids, unicellular organisms encased in calcified scale-like structures, the coccoliths, which are readily preserved in marine sediments and have for a long time been important indicators in micropalaeontological studies. In the middle of this century it was recognized that there was a need to compile and standardize the terminology used in coccolith morphology (Braarud et al., 1955; Halldal & Markali, 1955). This approach was continued by several authors (e.g. Hay et al., 1966; Okada & McIntyre, 1977; Tappan, 1980; Perch-Nielsen, 1985) in published articles, and in the report from a Round Table session at the Rome 1970 Plankton Conference (Farinacci, 1971), which included terms from both fossil and extant taxa. Over the last two decades many new terms have been introduced as observations on coccolith morphology have improved through the use of the electron microscope, and recent glossaries covering various aspects of haptophyte terminology have been published by Heimdal (1993), Kleijne (1993) and Margulis et al. (1993).