Chronic exposure to UV radiation (UVR) is the major etiologic factor in the development of human skin cancers including squamous cell carcinoma (SCC). We have shown that protein kinase Cε (PKCε), a Ca²⁺-independent, phospholipid-dependent serine/threonine kinase, is an endogenous photosensitizer. PKCε is among the six isoforms (α, δ, ε, η, μ, and ζ) expressed in both mouse and human skin. PKCε transgenic mice, which overexpress PKCε in the basal epidermal cells and cells of the hair follicle, are highly sensitive to UVR-induced cutaneous damage and development of SCC. We now present that PKCε-overexpressing, but not PKCδ-overexpressing, transgenic mice, when exposed to a single (4 kJ/m²) or repeated (four doses, 2 kJ/m²/dose, thrice weekly) UVR, emitted by Kodacel-filtered FS-40 sun lamps, elicit constitutive phosphorylation of signal transducers and activators of transcription 3 (Stat3) at both Tyr705 and Ser727 residues. UVR-induced phosphorylation of Stat3 accompanied increased expression of Stat3-regulated genes (c-myc, cyclin D1, cdc25A, and COX-2). In reciprocal immunoprecipitation/blotting experiments, phosphorylated Stat3 coimmunoprecipitated with PKCε. As observed in vivo using PKCε knockout mice and in vitro in an immunocomplex kinase assay, PKCε phosphorylated Stat3 at Ser727 residue. These results indicate for the first time that (a) PKCε is a Stat3Ser727 kinase; (b) PKCε-mediated phosphorylation of StatSer727 may be essential for transcriptional activity of Stat3; and (c) UVR-induced phosphorylation of Ser727 may be a key component of the mechanism by which PKCε imparts sensitivity to UVR-induced development of SCC. [Cancer Res 2007;67(3):1385–94]