Details of a series of three point flexure tests on 101.6 × 101.6 × 6.4 mm Wide Flange (WF)-section, pultruded Glass Reinforced Plastic (GRP) beams are presented. The beams were tested at four spans (L/d=21–45) under four sets of end conditions: clamped, bolted web and flange cleats, bolted web cleats and simply supported. Mid-span deflections and beam end rotations were recorded in the tests. The observed deflections and end rotations are compared with semi-rigid shear deformable beam theory predictions. Reasons are given for the differences (not large in the majority of cases) between the observed deflections and end rotations and the theoretical predictions. It is shown that deflection reduction and load enhancement performance indices provide a better basis for comparison of the experimental results and the theoretical predictions. The comparisons demonstrate and quantify the improvements in structural performance, arising from the inherent rotational stiffness of practical end connections, which could be utilised at no extra cost by adopting a semi-rigid design philosophy.