An analysis of changes in the period of the quasi-biennial oscillation (QBO) in the zonal velocity of the equatorial stratospheric wind on isobaric surfaces 70, 50, 40, 30, 20, 15, and 10 hPa for 1953--1997 is made. In particular, wavelet analysis, high-resolution spectral analysis, and circle map analysis have been made. Different methods reveal that the QBO in the equatorial wind velocity at the noted stratospheric levels manifests itself mainly as two prevailing regimes with periods of about 2 and 2.5 years alternating with each other. At middle-stratosphere levels the QBO amplitude changes as well with the changing period (increases with the period increase). The two periods are in a rational ratio to the annual period (with the ratios of corresponding frequencies to the annual frequency equal to one half and two fifths), thus pointing out the phase locking of the QBO to the annual cycle. Spectra of wind velocity exhibit, in addition to the main QBO periods, the annual and semiannual oscillations as well as 20-, 8-, and 8.6-month oscillations corresponding to oscillations with combination frequencies equal to the difference and sum of the annual frequency and the main QBO frequencies. Hypothetical conceptual mechanisms of the two regimes of the QBO are discussed. ¿ 2000 American Geophysical Union