Highly (001) oriented thin films of Co2MnSi have been grown on lattice-matched GaAs(001) without a buffer layer. Stoichiometric films grown at the highest substrate temperature of 689 K showed the lowest resistivity (33 μΩ cm at 4.2 K) and the lowest coercivity (14 Oe). Twofold in-plane magnetic anisotropy was observed due to the inequivalence of the <110> directions, and this was attributed to the nature of the bonding at the reconstructed GaAs surface. Interfacial reactions resulted in the formation of an epitaxial Mn-As region and a thin interfacial layer that was Co-Ga rich. This prevented the desired lattice matching and resulted in films with a saturation magnetization slightly below the bulk value. In spite of this, the spin polarization of the free surface was measured to be 55%, similar to bulk material.