The Neurospora protein kinase C (NPKC) is a regulator of light responsive genes. We have studied the function of NPKC in light response by investigating its biochemical and functional interaction with the blue light photoreceptor white-collar 1 (WC-1), showing that activation of NPKC leads to a significant decrease in WC-1 protein levels. Furthermore, we show that WC-1 and NPKC interact in a light-regulated manner in vivo, and that protein kinase C (PKC) phosphorylates WC-1 in vitro. We designed dominant negative and constitutively active forms of PKC which are able to induce either a large increase of WC-1 protein level or a strong reduction respectively. Moreover, these changes in PKC activity result in an altered light response. As WC-1 is a key component of Neurospora circadian clock and regulates the clock oscillator component FRQ we investigated the effect of NPKC-mutated forms on FRQ levels. We show that changes in PKC activity affect FRQ levels and the robustness of the circadian clock. Together these data identify NPKC as a novel component of the Neurospora light signal transduction pathway that modulates the circadian clock.