During winter the temperate/subpolar calanoid copepod Calanus finmarchicus enters seasonal diapause, a type of dormancy, to overcome this period of low food availability. Daily and seasonal rhythms of zooplankton might be under the control of an endogenous circadian clock ensuring optimal synchronizatzion of physiological, biochemical and behavioral processes to prevailing local environmental conditions. Photoperiod (daylength) is supposed to be the most reliable entrainment cue of an animals seasonal cycle for synchronization with the environment. A small timing mismatch between biological processes and the environment such as temporal shifts of the onset of phytoplankton blooms caused by climate change could potentially have severe consequences for the entire Calanus-based ecosystem. Nevertheless, limited knowledge is available concerning the synchronization of C. finmarchicus and marine organisms inhabiting polar regions with their environment. This study aimed to investigate the performance of the clock at distinct times during diapause to gain knowledge concerning the role of the clock in seasonal diapause of C. finmarchicus. Thus, diurnal clock gene expression patterns in C. finmarchicus being in early (September 2014, 10 h L: 14 h D) and late (January 2015, DD) diapause. Copepods have been collected by 24 h in situ sampling from Kongsfjorden, Svalbard (78.6°N, 11.6°E). Primers were designed for recently described potential clock genes (cry1, cry2, clk, cyc, per1, tim, dbt2, vri) in C. finmarchicus. Clock gene expression patterns were analyzed with Real-Time quantitative PCR. We could show that most clock genes showed a diel rhythmic oscillation during early diapause (LD), whereas in late diapause (DD) a significant rhythmic oscillations was not detectable for most of the investigated genes. Comparison of early and late diapause between each diel time point revealed significant differences. Overall, copepods caught in early diapause had higher relative mRNA levels compared to copepods sampled in January. These findings indicate a diurnal clock in C. finmarchicus. This might be the first sign of a circadian clock in C. fi•nmarchicus and the potential involvement of the clock in seasonal diapause. Further studies need to investigate diurnal protein levels and clock-associated genes to get an understanding of the interplay of clock genes, photoperiod sensing and diapause in C. finmarchicus