We develop a model for deformation in an extending continental lithosphere that is stratified in density and strength, assuming a rheology consistent with seismic focal depths and experimetal flow laws. The model demonstrates that necking instabilities at two wavelengths will arise due to the presence of a strong upper crust and upper mantle separated by a weak lower crust. The magnitudes of the instabilities are directly related to strength contrasts within the lithosphere, while the dominant wavelengths of necking are controlled mainly by the thickness of the strong layers. The results are applied to the Basin and Range Province of the western United States where two scales of deformation can be recognized, one corresponding to the spacing of ranges and the other to the width of tilt domains. A Bouguer gravity anomaly and associated regional topography with a wavelength comparable to the width of tilt domains has also been recognized. For plausible density and strength stratifications, our results show that the horizontal scale of short wavelength necking is consistent with the spacing of individual basins and ranges, while that of the longer wavelength necking is consistent with the width of tilt domains. We thus suggest that Basin and Range deformation may be controlled by two scales of extensional instability. Extension in the weak lower crust in this model is laterally displaced from regions of upper crustal extension. The resultant horizontal shearing in the lower crust may be a mechanism for the initiation of low-angle extensional detachments.