The principal streak of reflection high‐energy electron diffraction (RHEED) patterns of (100) GaAs grown by molecular‐beam epitaxy (MBE) has been studied with an advanced imaging system which uses a charge‐coupled device (CCD) camera and an array processor. Calculations by Fernandez suggest that the profile of the principal streak from MBE GaAs should be very sensitive to the relative coverage of Ga and As, as well as the degree of roughness of the GaAs surface. Observations of the principal streak profile under static and dynamic conditions confirm qualitatively the theoretical calculations of Fernandez. The principal streak profile changes dramatically as the relative coverage of Ga and As is changed. At approximately equal Ga and As coverage, the intensity at the specular position is relatively low, and increases as the degree of Ga or As coverage increases. Significant changes in principal streak intensity and shape are produced by changes in adatom coverage as small as 0.1 monolayer (ML), and these changes saturate as the surface reaches full Ga or As coverage. The streak profile is noticeably different for Ga‐ and As‐stabilized surfaces, indicating that the (2×4) and (4×2) reconstructions are tetragonally distorted. Furthermore, the relative coverage of Ga and As adatoms during growth is strongly dependent on the ratio of the Ga and As fluxes. The lack of strong damping of oscillations in the specular spot intensity under optimized growth conditions is probably the result of relatively high Ga coverage and therefore higher Ga mobility on partially completed monolayers.