Single coccoliths of Emiliania huxleyi grown in mesocosm enclosures (60°16′N, 05°14′E, May–June 1991) under different N[ratio ]P regimes were analysed in a scanning electron microscope. The results indicate that only E. huxleyi with Type A coccoliths was present in the enclosures. Approximately 80–90% of the total coccolith assemblages had developed normally, whilst the remainder were malformed, incompletely grown or dissolved. Severely under-calcified specimens were rare and dissolution and breakage less than 5%. The coccoliths were of larger size than normal, as has been found previously in fjords of southwestern Norway, supporting the conclusion that a local population of E. huxleyi has developed, specific to these waters. Both phosphorus and nitrogen stress caused significant changes in coccolith size and evidence of malformation was clear, particularly in the low-phosphate enclosure. Although the observations presented here concern only Type A coccoliths and it is not known how nutrient stress may affect the coccoliths of the other types of E. huxleyi, they do serve to stress the fact that environmental conditions may possibly obscure genetically determined features. Following our observations on coccolith morphology in relation to nutrient status in enclosures, it will be of interest to test whether a similar correlation can be detected in the natural environment.