The lattice structure of defects in undoped, semi-insulating liquid encapsulated Czochralski GaAs has been studied using conventional and high resolution transmission electron microscopy. The material exhibits a cellular structure along the less than 100 greater than growth direction. The central portion of the cells is virtually free of extended defects. In the vicinity of the cell boundaries, large densities of various defects are present, among them dislocations and small precipitates. Precipitates ranging from less than 1 nm to about 20 nm in diameter and with a different structure than that of GaAs have been observed using lattice imaging techniques. In addition, small microdefects with typical diameters between 2 and 3 nm have been observed. The latter appear to be small loops lying along (111) planes, of both the vacancy and the interstitial types.