A skin size (k⊥ ≲ ωpe/c) plasma mode characterized by a dispersion relation ω ≃ ck⊥k∥/kDe (kDe the electron Debye wavenumber), adiabatic ions and adiabatic electrons ω ≪ k∥vTe in a uniform plasma is destabilized in tokamaks by a modest electron temperature gradient (ETG) ηe and ballooning parameter αe. The instability is of hydrodynamic (ballooning) nature and thus when unstable, a large electron thermal diffusivity emerges because of the longer cross-field wavelength than the case of the ETG mode.