  We report resistivity measurements from 0.03 K to 10 K in a dilute high
mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a
gated field-effect transistor geometry, a wide range of densities, $0.16 \times
10^{10} {cm}^{-2}$ to $7.5 \times 10^{10} {cm}^{-2}$, are explored. For high
densities, the results are quantitatively shown to be due to scattering by
acoustic phonons and impurities. In an intermediate range of densities, a peak
in the resistivity is observed for temperatures below 1 K. This non-monotonic
resistivity can be understood by considering the known scattering mechanisms of
phonons, bulk and interface ionized impurities. Still lower densities appear
insulating to the lowest temperature measured.
