Epitaxial layers of GaAs with peak mobilities as high as 200 000 cm 2/V s at 50 K have been grown in an atmospheric pressure organometallic vapor phase epitaxy reactor using trimethylgallium (TMG) and arsine. The growth conditions which lead to high-mobility GaAs are described in this letter. Low-temperature photoluminescence and temperature-dependent Hall measurements are used to study the dependence of the incorporation of residual impurities on the growth temperature and arsine partial pressure. Carbon acceptor densities <1014 cm-3 were measured in the highest purity samples.