Analysis of the solar radio zebra-pattern (ZP) spectrum for the burst on 21 June 2011 has shown that the frequencies corresponding to the stripes of this ZP experience quasiperiodic oscillations relative to some average values. The period of such oscillations, expressed in the number of the ZP stripes, is 2.41±0.21, and expressed in frequencies, it is (5.00±0.68) MHz. The change in the period of oscillations with time anticorrelates with the amplitude of the oscillations. The values of the harmonic numbers for the corresponding bands are given, and thus the magnetic-field strength is also estimated on the basis of the theory of double plasma resonance (DPR). In addition, a possible change in the Lbh/Lnh parameter in the ZP-generation region is taken into account (Lbh and Lnh respectively are the magnetic-field and density scales). Calculations of the frequency-drift rate, carried out using an improved method for its determination, have shown that the drift values (3--8 MHzs−1) are in accordance with Kaneda et al. (Astrophys. J. Lett. 855, L29, 2018). By using two density models of the solar atmosphere, the wavelength of these oscillations has also been determined. For the model presented by Aschwanden (Space Sci. Rev. 101, 1, 2002), the wavelength is about 1370 km while for the barometric density model, the wavelength is about 4650 km. The wavelength increases with time; for example, in the first model, the wavelength increases with time from 1200 to 1490 km. The calculated kink and sausage wave velocities turned out to be significantly lower than the observed ones. The reason for this discrepancy requires additional analysis.