The earth's lithosphere and asthenosphere are modeled as a thin elastic sheet and a fluid substratum, respectively; the physical principles involved are briefly described. The flexural rigidity of the lithosphere is deduced from observations of the wavelength and amplitude of bending in the vicinity of supercrustal loads. Data from Lake Bonneville given by M. D. Crittenden, Jr., are reinterpreted to give a value for the flexural rigidity of the lithosphere in the Basin and Range province of the western United States of 5 × 10²² Newton meters. Observations of loading in Canada give values for the flexural rigidity of greater than 3 × 10²⁰N m for the Caribou Mountains in Northern Alberta; about 4 × 10²³ N m for the topography over the Interior Plains; about 10²³ N m for the Boothia uplift in arctic Canada; and about 10²⁵ N m for the bending of the beaches of Pleistocene Lakes Agassiz and Algonquin. The flexure of the lithosphere at Hawaii and the bending of the oceanic lithosphere near island arcs give values of about 2 × 10²³ N m. For short-term loads (10³−10⁴ years) the flexural rigidity of the continental lithosphere is almost two orders of magnitude larger than for long-term loads, indicating nonelastic behavior of the lithosphere with a viscous (about 10²³ N sec m⁻²) as well as an elastic response to stress. From the values of the flexural rigidity, the thickness of the continental lithosphere is inferred to be about 110 km and that of the oceanic lithosphere about 75 km or more. The anomalously low flexural rigidity of the lithosphere of the Basin and Range province may be due to a very thin lithosphere, only about 20 km thick, with hot, lower crustal material acting as an asthenosphere.