Objective: The link between alterations in circadian rhythms and depression are well established, but the underlying mechanisms are far less elucidated. We investigated the circadian characteristics of immobility behavior in wild type (WT) mice and mice with mutations in core Clock genes. Methods: All mice were tested with forced swim test (FST) at 4 h intervals. Results: These experiments revealed significant diurnal rhythms associated with immobility behavior in both male and female WT mice with sex-different circadian properties. In addition, male mice showed significantly less immobility during the night phase in comparison to female mice. Female Per1(Brdm1) mice also showed significant rhythmicity. However, the timing of rhythmicity was very different from that observed in female wild type mice. Male Per1(Brdm1) mice showed a pattern of rhythmicity similar to that of wild type mice. Furthermore, female Per1(Brdm1) mice showed higher duration of immobility in comparison to male Per1(Brdm1) mice in both daytime and early night phases. Neither Per2(Brdm1) nor Clock(19) mice showed significant rhythmicity, but both female Per2(Brdm1) and Clock(19) mice had lower levels of immobility, compared to males. Conclusions: This study highlights the differences in the circadian characteristics of immobility induced by FST in WT, Clock(19), Per1, and Per2 deficient mice.