The analysis of the complex network of signal transduction chains has demonstrated the importance of transcription factor activities for the control of gene expression. To understand how transcription factor activities in plants are regulated in response to light, we analyzed the common plant regulatory factor 2 (CPRF2) from parsley (Petroselinum crispum L.) that interacts with promoter elements of light-regulated genes. Here, we demonstrate that CPRF2 is a phosphoprotein in vivo and that its phosphorylation state is rapidly increased in response to light. Phosphorylation in vitro as well as in vivo occurs primarily within the C-terminal half of the factor, and is caused by a cytosolic 40-kDa protein serine kinase. In contrast to other plant basic leucine-zipper motif factors, phosphorylation of CPRF2 does not alter its DNA binding activity. Therefore, we discuss alternative functions of the light-dependent phosphorylation of CPRF2 including the regulation of its nucleocytoplasmic partitioning.