The elastic buckling modes of pultruded I-beams subjected to various loading conditions are studied. The coupling of lateral and distortional buckling, very likely to appear in thin-walled cross-sections, is investigated. Plate theory is used to allow for distortion of the cross-section. Shear effects and bendingt\\isting coupling are accounted for in the analysis, because of their significant role. The effect of fiber orientation in the matrix and volume fraction is investigated by a parametric study. Pultruded cross-sections are always.thin-walled due to constraints in the manufacturing process. Hence, the buckling strength determines the overall strength of the member. The results presented correspond to actual pultruded cross-sections being used in civil engineering types of structure.