We report the temperature dependence, between 20° and 200°C, of free carrier impact ionisation rates in GaAs along the 〈100〉, 〈110〉 and 〈 111〉 directions for the electric field range 4.5 × 105 V/cm. These results should be of interest for the design of avalanche devices such as Impatt diodes which have high operating temperatures.© 1979, The Institution of Electrical Engineers. All rights reserved.