Epitaxial layers of GaAs have been grown by metalorganic molecular beam epitaxy (MOMBE) with atomic carbon concentrations ranging from 4×10 17 to 3.5×1020 cm -3. The dependences of GaAs lattice parameter and hole concentration on atomic carbon concentration have been determined from x-ray diffraction, Hall effect, and secondary-ion mass spectrometry measurements. For atomic carbon concentrations in excess of 1×1019 cm-3, the hole concentrations are less than the corresponding atomic carbon concentrations. Lattice parameter shifts as large as 0.2% are observed for carbon concentrations in excess of 1×10 20 cm-3, which results in misfit dislocation generation in some cases due to the lattice mismatch between the C-doped epilayer and undoped substrate. Over the entire range of carbon concentrations investigated, Vegard's law accurately predicts the observed lattice contraction.