Long-period seismic data (longer than about 100 s) are sensitive to density as well as P wave and S wave structure in the Earth. Seismograms containing overtone and fundamental mode signals are inverted directly for density and S wave velocity structure. A simple relation between P wave and S wave velocity perturbations is assumed in order to reduce the number of unknowns. A model is parameterized by spherical harmonics up to degree and order 8 and by five knots in depth (at 30, 220, 450, 670 and 882 km) with linear interpolation between adjacent knots. Density resolution is good only at shallow depths; in fact, only the average in the upper 200 km is marginally resolved. S wave resolution is generally good throughout the depth range. S wave results show good correlation to surface geological features and are similar to previous studies. Density results show quite different patterns from S wave maps; high-density regions appear along the subduction zones surrounding the Pacific Ocean and low-density regions appear in most major continents. Most continents have low density and high S wave velocity, which suggests that continental mantle consists of compositionally different materials since thermal effects can never produce such perturbations. High S wave velocity also suggests a relatively cold geotherm for continents, and thus continents should consist of significantly low-density material such as depleted mantle rocks, as proposed by many researchers before. The density map also correlates well with a geoid map for spherical harmonic components l=4--6 but not for l=2,3. It suggests that sources for these geoid undulations l=4--6 are in the upper mantle, probably in the upper 300--400 km, but those for l=2,3 are in deeper regions. ¿1991 American Geophysical Union