  We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs
heterostructure containing a two-dimensional electron system (2DES) in a
low-density regime ($<$ $10^{10}$ cm$^{-2}$) at zero magnetic field. The
interaction of the surface acoustic wave with the 2DES is not well described by
a simple model using low-frequency conductivity measurements. We speculate that
this conflict is a result of inhomogeneities in the 2DES which become very
important at low density. This has implications for the putative
metal-insulator transition in two dimensions.
