Analysis of rawinsonde and rocketsonde data at Ascension Island (7.6 ¿S, 14.4 ¿W) and Kwajalein (8.7 ¿N, 167 ¿E) in 1962--1991 suggests that the quasi-biennial oscillation (QBO) in the middle stratosphere is synchronized with the seasonal cycle and that descending westerly phases of the stratopause semiannual oscillation (SAO) are strongly influenced by the underlying QBO. The effect of the seasonal cycle on the QBO in the middle stratosphere is revealed in two, perhaps unrelated, observations: first, a tendency for deseasonalized QBO westerly maxima to occur in local winter (or to avoid local summer); second, a smooth, uninterrupted connection between descending SAO westerly shear zones and the formation of a new QBO westerly shear zone aloft. The timing of deseasonalized QBO westerly maxima in the middle stratosphere allows a simple composite of 2- and 3- year cycles to be constructed from the data, illustrating the effect of the QBO on descending westerly phases of the stratopause SAO. ¿ 1997 American Geophysical Union