Details of a series of lateral buckling tests on a pultruded GRP I-section cantilever beam are described. The cantilever, which had spans ranging from 0.5 to 1.5 m, was subjected to point loads applied above, at and below the centroid of the free end cross-section. Vertical and horizontal deflections and the rotation of the free end cross-section were recorded as the beam was loaded and unloaded. Lateral buckling loads computed from modified simple closed form formulae and finite element eigenvalue analysis were compared with the maximum test loads. The computed buckling loads were generally significantly lower than the maximum test loads, though the differences between the loads appeared to be smallest for the longest span with the load applied just above the lower flange of the free end cross-section. The tests showed that significant vertical free end deflections develop before the cantilever buckles laterally and it is concluded that the effects of prebuckling deformations and geometric nonlinearity in addition to a more accurate evaluation of the in-plane shear modulus may have to be taken into account in order to achieve a closer correlation between the numerical analysis and the test results.