Neuronal Ca2+/calmodulin-dependent protein kinase II (CaMKII) is one of the most abundant protein kinases in the brain, and phosphorylates a broad range of substrate proteins. The phosphorylation of microtubule tau by CaMKII was investigated using tandem mass spectrometry (MS/MS). Recombinant human tau was phosphorylated at Thr212, Ser214, Ser262, and Ser356 by CaMKII. The phosphorylation of these sites is found in paired helical filament (PHF)-tau. In addition to these sites, Ser131 and Thr135 were phosphorylated by CaMKII. Phosphorylation at Ser131, Thr135, Thr212 and Ser214 by CaMKII has not been reported previously. Thr212 and Ser214 are in the consensus phosphorylation sequence of CaMKII (RXXS/T), and non-fetal-type phosphorylation sites of tau. Non-fetal-type phosphorylation may produce PHF-tau. These results suggested that CaMKII is involved in the phosphorylation of tau in Alzheimer's disease brain.