Frequency spectra of axially symmetric extensional vibration of an isotropic disk plate have been calculated for the critical value, 1/3, of Poisson’s ratio σ of the disk material, and the spectra have been compared with the earlier works on spectra for σ<1/3 (0.18) and those for σ≳1/3 (0.37). In spite of absence of the imaginary loop in the present dispersion curve, there appears to be no drastic change in spectral pattern. The frequency of the present spectra increases rather rapidly with decreasingr(diameter/thickness ratio), showing no thickness dependence even near the normalized cutoff frequencyk. It is suggested from the present spectra that disk material with σ of 1/3 is not suitable for application of disk in the thickness extensional mode.