The production of disorder as a function of incident ion fiuence has been studied for various ion species on (100) GaAs at 40 K.In situmeasurements of damage were performed, using the Rutherford backscattering-channeling technique with 2 MeV He+ions. Also the effects of disorder production by the He+analysing beam on both unirradiated crystals of GaAs and on crystals previously damaged with various fluences of 40 keV N+and Bi+and 80 keV N2+have been carried out. For initial damage levels up to ∼50 % of saturation damage the He+disorder production rate was found to increase with initial heavy ion damage level. In a similar experiment using 0.5 MeV He+ions on a crystal previously irradiated with 40 keV N+, increased damage production was found which scaled approximately with the increased elastic deposited energy. Results of the study of heavy ion damage production using 20–200 keV Sb+, 40 keV N+, P+, As+, Bi+and the corresponding 80 keV diatomic implants in GaAs are discussed in terms of different cascade density regimes. In all cases, the measured disorder is greater, by up to an order of magnitude, than predicted by binary collision theory. For the heavy ion irradiations, a linear build-up of damage with fiuence is observed. For lighter ions, a superlinear dependence of damage with fiuence is observed at intermediate fluences (i.e., when the average damage level is ≳ 10% of saturation). The observed behaviour is attributed to a transition from crystalline to amorphous structure when local defect density exceeds some critical value.