Admittance estimates from line-of-sight (LOS) acceleration profiles of the Mars Global Surveyor spacecraft are used to constrain the mean crustal thickness and surface density centered on the hemispheric dichotomy, from 110¿E to 220¿E, 40¿S to 20¿N. Models with uniform crustal properties predict lower than expected bulk crustal densities. Two-layer models with loading only at the surface and the Moho produce satisfactory fits to the data. The best fit surface density, crustal thickness tc, and elastic thickness Te are 2.5 Mg m-3, 27 km, and 61 km, respectively. Higher elastic thicknesses require lower crustal thicknesses, and vice versa. The best fit ratio of Moho to surface loading F is close to 1. Models with no bottom loading (F = 0) provide a poor fit to the data; underestimates in F result in underestimates of both tc and Te. The surface density is lower than that measured from Martian meteorites and by admittance analyses of young volcanoes but is well constrained by the short-wavelength admittance values. For misfits up to 1.5 times the minimum value and a fixed surface density of 2.5 Mg m-3, the ranges of tc, Te, and F are 1--75 km, 37--89 km, and 0.4--2.6, respectively. The apparently compensated nature of the large impact basins has been used to infer a lower bound on southern hemisphere crustal thickness of 45 km. If this estimate is correct, the likely mean crustal thickness in the area considered is 55 ¿ 20 km.