A band-pass filter is used to obtain an estimate of the seasonal CO2 cycle at Mauna Loa from monthly mean concentration data. The signal that is extracted shows interannual variations in the amplitude with, as has frequently been noted, a preponderance of larger amplitudes in recent years. An interesting feature of the signal is that abnormally small amplitude peaks in the (northern) spring are almost always followed by small-amplitude troughs in the following fall but that there is no similar correlation between the amplitudes of peaks in fall and the following spring. This suggests that the amplitude variations reflect changes in individual northern growth seasons. ¿American Geophysical Union 1987