The marine copepod Calanus finmarchicus is an important key species in the Northern Atlantic due to his abundance and his position in the food web. It performs diel vertical migration (DVM), staying in deeper water layers during the day and ascending to the surface in the night. The exact trigger for the DVM is not known yet, but light seems to have an important influence on the position of C. finmarchicus. Some studies suggest an involvement of an endogenous rhythm, which controls the vertical position of C. finmarchicus throughout the day. In this work the DVM and respiration rate of C. finmarchicus were examined under natural simulated light conditions to identify possible circadian rhythms. Therefore, two laboratory experiments were performed with the CV-stage of C. finmarchicus under light/dark (LD) and constant darkness (DD) conditions. The position of C. finmarchicus in the DVM experiment showed a clear diurnal rhythm, with significant differences between day and night. The rhythm persisted in weaker form during constant darkness, indicating that an endogenous circadian clock is involved in the DVM. The results from the respiration experiment supported the assumption, revealing a rhythmicity in the oxygen uptake that also persisted under constant darkness. The light seemed to have in both experiments the role of a Zeitgeber that synchronises the circadian clock. For a final identification of the assumed clock a genetic analysis is necessary. However the experiments showed evidence that the DVM and the metabolic activity of C. finmarchicus are controlled by a circadian clock.