InSb films grown directly on (100) GaAs substrates by MBE have been examined in the transmission electron microscope. High quality, epitaxial layers were deposited despite the 14.6% lattice mismatch between film and substrate. Nearly all of the misfit strain has been accommodated by a square array of a/2{011} edge-type misfit dislocations spaced on average 29Å apart. These defects are proposed to be spontaneously generated in the epitaxial layer almost as soon as it begins to grow, and are favoured over 60° type dislocations because they are more efficient at relieving misfit strain while allowing more coherent interface area to form. The films that have been produced have low defect densities (ie. threading dislocations and microtwins) considering the large lattice mismatch in this system. Antimony precipitates have been noted in some films, but can be totally eliminated by careful control of the In/Sb flux ratio. Finally we have observed that loss of Sb from the specimen surface leads to a gradual degradation of the InSb during TEM observation, leading to the appearance of surface indium or indium oxide.