The circadian clock modulates the expression of approximately one-third of Arabidopsis genes and as such plays a central role in the regulation of plant metabolism and responses to the environment. We have recently identified a novel component of the Arabidopsis circadian clock, JMJD5, based on its coexpression with TOC1, an evening-phased component of the molecular oscillator. We now examine the genetic interaction between TOC1 and JMJD5 in greater detail and demonstrate that toc1 is not epistatic to jmjd5, suggesting that these two proteins act in closely linked but parallel genetic pathways. The human homolog of JMJD5, KDM8, has been shown to have histone demethylation activity and is able to partially rescue the plant jmjd5 circadian phenotype. The potential role of JMJD5 as a histone demethylase within the circadian clock is discussed.