Results of a series of tests on rectangular cross-section pultruded GRP cantilever beams are presented. The beams are loaded in their stiffer plane of symmetry by means of a single point load acting through the centroid of the cross-section at the free end. Each beam was loaded well into the lateral postbuckling regime and then unloaded. Throughout the loading and unloading regimes end displacements, i.e. horizontal and vertical translations and the rotation of the cross-section, were recorded. Plots of support moment versus free end rotation are presented for all 16 beams tested and critical support moments (calculated using a modified formula for the critical moment of an isotropic cantilever) are compared with experimental maximum moments. The maximum support moments applied in the tests are also compared with theoretical critical support moments for span to depth ratios ranging from 4 to 25. It is shown that critical support moments provide reasonable lower bound estimates of the maximum support moments provided the span to depth ratio, l/d, exceeds 10.