Of the many plant genes whose expressions are controlled by the circadian clock, one of the phosphoenolpyruvate carboxylase kinase genes in soya bean (Glycine max) exhibits the unusual property that its control is organ-specific--it is under circadian control in leaves but not in roots. Preliminary experiments suggest that the same is true for at least one gene in Arabidopsis thaliana. It will be important to define the extent and function of this phenomenon and the underlying mechanism.