A reverse plane bending fatigue machine of simple construction is described which has proved effective in testing a range of materials either in controlled atmosphere or in vacuo (i.e. gas pressures down to approximately 1 μtorr). The advantages of the machine over other types described in the literature are discussed in terms of the stability of the strain cycle, the simplicity of the deflecting mechanism, the ease of metallographic observations on the specimen surface, and the importance of an impermeable specimen enclosure.