Twenty high-strength plain concrete deep beams were tested under torsion. The variables were concrete strength and depth/width (aspect) ratio. Concrete strengths of 51.01, 58.53, 76.60 and 83.66 MPa and depth/width (aspect) ratios varying from 1 to 5 were used. The tested beams failed suddenly and violently along a smooth surface. The crack inclinations on the beam surfaces were influenced by the concrete strength and aspect ratio. Test results show that by increasing the concrete strength or by reducing the aspect ratio, the torsional capacity and the beam stiffness increases while the angle of twist decreases. A modification has been suggested to the strength equation of the ACI Code 318-89 for normal-strength concrete shallow beams under torsion to include the effect of high-strength concrete for deep beams. The proposed equation gave .a. good estimate of the experimental torsional strength
