Recent observations and theoretical work suggest that the quasi-2-day planetary wave in the summertime mesosphere is composed of multiple superposed zonal wavenumbers. Here we use EOS Aura MLS temperature data to determine the component zonal wavenumbers of the quasi-2-day wave in the mesosphere at latitudes of 70S to 70N from 2004 - 2009. We consider the effect of aliasing between different wavenumbers and note that significant aliasing can occur and result in spurious signals, particularly at high latitudes in winter. The seasonal evolution of the different wavenumbers is investigated and found to be very different between the Northern and Southern Hemispheres. In both hemispheres the wave is dominated by westward travelling waves of zonal wavenumber 3 and 4 (W3 and W4). However, in the Southern Hemisphere the wave is dominated by the W3 component but in the Northern Hemisphere the W3 component is smaller and the W4 component is often of similar or larger amplitude. A small-amplitude westward travelling zonal wavenumber 2 (W2) wave is also evident in both hemispheres. In the Northern Hemisphere, the W2 amplitudes never exceed 3 K, the W3 amplitudes can reach 3.5 K and the W4 can be the largest component reaching amplitudes of 4 K. In the Southern Hemisphere, the W2 amplitudes can reach up to 3.5 K, the W3 amplitudes can be much larger, reaching 12 K, and the W4 amplitudes are smaller than in the Northern Hemisphere, in 4 out of 5 years not exceeding 3 K. The Northern Hemisphere W4 can reach large amplitudes in August when the W3 is small which means that the late summer Northern Hemisphere quasi-2-day wave is usually a W4 oscillation rather than the familiar W3. In contrast, in the Southern Hemisphere, the W3 is often larger than the W4 around the the summer solstice and there are no episodes observed where the wave becomes dominated by the W4 for an extended period of time. A high degree of inter-annual variability is evident, particularly in the Southern Hemisphere, where the W3 peak amplitudes vary from 12 K in January 2006 to 3 K in January 2009. The height-latitude structure of the W4 suggests that this wave is a (4, 0) Rossby-gravity wave.