@@ The endogenous circadian clock confers an internal periodicity that synchronizes an organism with the environmental period imposed by Earths rotation; when internal and external periods diverge, performance measures such as net photosynthesis et al decline dramatically. In plants, both transcriptional and posttranslational mechanisms are prominent in the basic network architecture of circadian systems. In Arabidopsis thaliana, as in most other eukaryotes studied to date, the circadian clock includes multiple interlocked feedback loops. Here we report two putative transcriptional factors, SA5, and its partially redundant homolog CS1, play critical roles in circadian clock. Mutation of SA5/CS1 leads to the lengthened circadian period.