GaAs nanowires have been grown on single-crystal (001) Si and GaAs substrates using the vapour-liquid-solid (VLS) mechanism in a MOCVD chamber. These nanowires typically exhibited a circular cross-section with diameters in the range 10-40 nm. They contain a large density of growth twins that are here investigated in detail using transmission electron microscopy and high-resolution electron microscopy. Results indicate that the wires predominantly have the zinc blende diamond cubic structure with the growth axis parallel to 〈111〉 and twins perpendicular to the growth axis. In many cases, multiple twins tend to cluster in groups along the length of the nanowires, either in a near-periodic fashion or on consecutive 111 planes, resulting in the formation of local regions exhibiting the hexagonal wurtzite structure of GaAs. The small difference in energy between these two forms of GaAs is likely to be the reason for the coexistence of both zinc blende and wurtzite structures within the same nanowire.