The seasonal moulting pattern of Calanus finmarchicus, related to body size, oil content and depth distribution, was investigated on the Norwegian midshelf from February to October 1997. Both gonad and gnathobase development were well correlated with time, and proved to be good indices for tracing the moulting cycle. The growth and development of gonads preceded the formation of new teeth. Both gonad development and tooth formation patterns in overwintering stageV copepodites (CV) indicated that the most intensive moulting occurred in spring. In the upper 200m layer, the proportion of copepods in premoult phases ranged from 10% at the beginning of February, 60–95% from the end of February through April, to 4–7% in mid-May. No significant latitudinally determined variations in the moulting pattern were observed. The upward migration of deep-dwelling overwintering CVs was not synchronous and was determined by the readiness to moult. In April, no CVs were found at depths >300m, that may indicate the termination of the overwintering moulting from Go. By the end of May, the large-sized, lipid-rich CVs of G1 started descending to depth. The second period of moulting (i.e. most likely the formation of G2) was only recorded to have taken place in the surface (0–200m) component of the CV stock from July onwards. This group was estimated to make up 10–20% of CVs inhabiting the upper 200m layer and to have originated from the smallest sized fraction of CVs. There was a consistent trend towards size-differential moulting from CVs, in both spring and summer. The large-sized CVs moulted into adults earlier in spring compared with the smaller ones. By mid-June, the large and lipid-rich CVs of G1 disappeared from the uppermost layers, sinking to depths. In summer (July onwards), the moulting of small CVs is probably the main contributor to G2 copepodites that completed the cycle through to CIV/CV by the end of October.