We report systematic variations in the fundamental period of seismic site resonance within the Mississippi embayment. The resonance periods are revealed by peaks in horizontal to vertical (H/V) spectral ratios of microtremor measurements, and increase with the depth to the sediment/basement interface. The sediment depth varies slowly enough spatially that at any site it can be modeled as a one-dimensional problem, revealing structural details of the sedimentary column. The strong impedance contrast and gentle dip of the base of the embayment sediments may trap seismic energy within the soft sediments, increasing durations and amplitudes of strong ground motions. Broadband recordings of microtremor within the Mississippi embayment provide a simple method to study the seismic response of the basin. ¿ 1999 American Geophysical Union