We use a technique to measure the spatial dynamics of Bloch wave packets in semiconductor superlattices and investigate the dependence of the dynamics on the optical excitation conditions. For excitations well above or below the center of the Wannier-Stark ladder (WSL), the wave packets perform harmonic oscillations following the prediction of Zener; for excitations near the center of the WSL, the wave packets undergo a symmetric oscillation with virtually zero center-of-mass amplitude.