Slices of pulled, n-type gallium arsenide crystals containing the growth axis have been examined in transmission using an infrared image converter. Under conditions of parallel illumination some samples exhibit a pattern of alternating dark and bright lines, whose spacing is sometimes equal to the length of crystal pulled during a rotation of the crucible and generally smaller. The visibility of these growth striations is shown to be due to periodic variations of the real part of the refractive index, arising from periodic variations of the width of the optical energy gap or of the free-carrier concentration. In very highly doped samples the band-gap changes are associated with a varying Burstein shift, largely compensated by a corresponding shrinking of the gap. In undoped crystals contaminated by silicon, carbon, or aluminum the band-gap changes can be ascribed to an alloy effect. Other inhomogeneities seen in transmission include strains (using polarized light), p-n and n-n junctions, a mottled appearance in boat-grown crystals and unidentified defects in epitaxially deposited layers. © 1966 The American Institute of Physics.