The quasi-2-day wave is a prominent dynamical feature of the mesopause region. The onset of the large-amplitude summertime 2-day wave in the southern hemisphere is characterized by a sudden increase in amplitude and by phase locking so that the wind maximum occurs at nearly the same local time from cycle to cycle. The phase locking suggests a linkage to a diurnally repeatable phenomenon. We suggest that the phase-locked 2-day wave is a nearly resonant wave forced parametrically by the zonally symmetric diurnal tides. Recent observations and modeling support significant tidal forcing. It may also be possible for the migrating tides to generate the 2-day wave nonlinearly through a set of mutually reinforcing interactions. The process involves the self-excitation of the 2-day wave by an interaction with a diurnal zonal wavenumber 6 oscillation, itself generated nonlinearly in a two-step process initiated by the interaction of the 2-day wave and the migrating tides. Sudden onset, large amplitude, and phase locking are all consistent with the instability of a wave that has become resonantly tuned to a subharmonic frequency of the tides by the background wind system. ¿ American Geophysical Union 1996