Effects of nitrogen (N) and phosphorus (P) limitation on calcification and coccolith production in Emiliania huxleyi were investigated in batch and chemostat cultures by means of chemical analyses of calcium (Ca) and organic carbon (C), counts of coccoliths and cells, and scanning electron microscopy. In a normally calcifying (C-cell) clone growing in the absence of nutrient limitation in batch or semi-continuous cultures, c. 36 attached plus detached coccoliths were present per cell. In batch cultures as well as in chemostats, the number rose to 70–120 when either N or P became strongly limiting. The molar Ca/C ratio, measured in chemostats, showed a corresponding increase from 1·07 to c. 1·38. The two types of limitation had opposing effects on the calcification of individual coccoliths: the mean Ca content of a coccolith decreased from a normal value of 0·60 pg to 0·46–0·49 pg under N limitation, and increased to 0·67–0·73 pg under P limitation. These effects were accompanied by corresponding modifications of coccolith morphology. In batch cultures of one naked (N-cell) clone grown to the stationary phase, P limitation but not N limitation induced the formation of c. 40 coccoliths per cell; these coccoliths were of aberrant shape. In another N-cell clone, P limitation had no such effect. The results suggest on one hand that coccolith formation is generally less dependent on N and P nutrients than is cell replication, and on the other hand that stages exist in the growth and calcification of a coccolith that are specifically influenced by either N- or P-containing cell constituents.