Doppler velocity data are used to study solar cycle variation of the frequencies of solar acoustic oscillations from 1977 to 1988. The results show a clear shift in frequency of the cross-correlation peaks of -0.37 + or - 0.04 microHz peak to peak as the solar activity cycle progresses from maximum to minimum. This effect is dependent on the l value of the modes. The results suggest that, as low l modes penetrate deeply into the sun's interior, there are structural changes correlated with the solar activity cycle.