Ground motion data recorded at 26 different locations in Las Vegas from six underground nuclear tests have been used to perform a preliminary microzonation of the city. The microzonation results are presented in the form of contour maps of the average observed seismic response throughout the city in 12 different frequency bands covering the period range from 0.16 to 6.0 sec. These data indicate that variations in long-period seismic response of as much as an order of magnitude can be expected at sites only a few kilometers apart in the city. Simple models of surface-wave propagation in the surface layer of alluvium of variable thickness can account for much of the observed variability. However, it appears that surface waves reflected back from the valley boundaries at the base of the surrounding mountain ranges also have a significant effect on the measured response.