DOLICHOMASTIX nummulifera gen. et sp. nov. (Prasinophyceae), was described by means of EM of whole mounts prepared in situ from wild material collected in the Canadian arctic, south Alaska [USA] and South Africa. Two other species [D. lepidota sp. nov. and D. eurylepidea sp. nov.] believed to belong to the same genus, but at present known only from South Africa, are recorded less fully. Members of the genus are characterized by possession of 2 long, flexible flagella and a relatively simple periplast, with minimal difference between body scales and flagellar scales. The exceptionally wide geographical distribution of the type species, when compared with those of the recently studied arctic coccolithophorids, is shown to pose some unusual problems needing further information in life history for effective evaluation. The zoids themselves are shown to be able to vegetate freely at arctic temperatures in summer (-1.degree. C in the middle of the Northwest Passage) and to tolerate the low salinity of parts of Hudson Bay from which coccolithophorids are excluded. Elsewhere, including the southern hemisphere, they are found in ecological conditions approximating to those of cool temperate seas. The world distribution is otherwise unknown.