Overwintering of C. finmarchicus and C. helgolandicus was studied in the field and in laboratory experiments in a shallow (120 m) Swedish fjord and in a deep (650 m) Norwegian fjord. In the Norwegian fjord 2 populations were found in late summer and autumn: in the surface layer the copepods were smaller and more active with high respiratory and digestive enzyme activities. The deep population, consisting of copepodid stage V (CV) and a few females, was torpid, had large oil sacs and empty guts. Their respiratory and digestive enzyme activities were very low. In the Swedish fjord CV in deep layers weighed much less than those in the Norwegian fjord. Weight-specific respiration was intermediate between deep and surface population in the Norwegian fjord. Overwintering copepodids do not feed. Metabolic rates allowed successful overwintering only in the Norwegian fjord. Experiments performed on several occasions during overwintering witnessed, in contrast to the situation in field populations, increased rates of respiration and molting. Mortality during molting and time span between capture and onset of molting decreased during autumn and winter. Internal development during overwintering is suggested.