The results are presented of investigations made on four cruciform biaxial fatigue specimens, each of which had a differently designed central region. This central region consisted of a reduced section of uniform thickness t over a circular area of diameter d which was connected by a transition radius to the full thickness of the cruciform. Equal tensile and compressive forces were applied alternately along one pair of arms only and the distribution of surface strain was studied by photoelastic-coating techniques after elastic cycling and then plastic cycling. To assess the fatigue performance of the specimens further cycling was carried out until cracks developed. Under elastic conditions, strains uniform over the thickness were calculated by finite-element techniques. For conventional fatigue tests in the high-endurance range the specimen with d/t = 12.5 is recommended. For high-strain low-endurance fatigue investigations the specimen with d/t = 5 is preferred. The recommended designs are free from problems of buckling and are such that an area of uniform strain is produced at the centre of the specimen.