Tau belongs to the microtubule-associated family of proteins that maintain cytoskeletal structure by regulating microtubule dynamics. In certain neurodegenerative diseases termed tauopathies, tau is abnormally phosphorylated and accumulates as filamentous inclusions. Transgenic mouse models that overexpress human tau have been widely used to investigate tau pathogenesis. Although many studies have attempted to elucidate the pathological function of transgenic human tau, it remains unknown whether endogenous mouse tau is involved in disease progression. Here we generated an mTau antibody that selectively recognizes mouse and rat tau, but not human tau. In rTg4510 tau transgenic mice, we identified a higher molecular weight mouse tau (~60-kDa) in sarkosyl-insoluble fractions. mTau antibody started to recognize intracellular aggregates and thread-like structures in 4- to 6-month-old rTg4510 mice. Tau inclusions appeared earlier, being detected in 2.5-month-old rTg4510 mice with MC1 antibody. Immunoelectron microscopy confirmed the presence of filamentous aggregates of mouse tau, which were abundant in oligodendrocytes but rare in neurons. Mouse tau inclusions in oligodendrocytes were confirmed by double-labeling with an oligodendrocyte marker. Our data indicate that mouse tau has potential aggregation properties in neurons and non-neurons. The mTau antibody will be useful for investigating the role of mouse tau in mouse models of tauopathy.