Accurate X-ray integrated-intensity data collected from an extended-face crystal of GaAs are analysed to provide detailed information on the thermal vibrations of atomic species, including cubic anharmonicity, at room temperature. The values obtained for the thermal parameters are BGa = 0.622 (3) A 2, B s = 0.483(5)A 2 and ~GaAs'-" --0.6(1) X 10-18jX -3 (defined in the text). The inclusion of cubic anharmonic thermal vibrations is shown to be highly significant. In order to interpret the data collected for certain low-angle Bragg reflections for which m h + k + l = 4n + 2 (in particular, 200, 222 and 222), it is necessary to consider bonding effects. It is shown that there is a net transfer of electron charge from gallium to arsenic [Q --0.12 (3) e] and that the inclusion of bonding effects in the least-squares analysis is highly significant. The analysis includes allowance for the extremely severe extinction effects present for such a perfect sample (minimum extinction factor 0.286). The refined value of the mean radius of © 1994 International Union of Crystallography Printed in Great Britain -all rights reserved perfect-crystal domains is 4.6 (2)I~m. The final fit, for 153 independent Bragg reflections, is excellent, as indicated by the weighted R factor of 0.683% and the goodness-of-fit parameter of 1.083. The results of the least-squares analysis are compared for the cases of relativistic Hartree-Fock, Thomas-Fermi-Dirac and relativistic Dirac-Slater atomic scattering factors, the former being favoured.