An investigation was conducted on a semi-monocoque box beam to determine the combined stress interaction curve for a corrugation-skin type of construction under combined compression and shear load. The investigation included the determination of initial buckling stresses and a comparison of measured stresses with computed stresses on the box beam. The interaction curve presented is most closely represented by the expression Rc + RS 1J = 1 where R is the appropriate stress ratio. Curves of effective width of skin versus corrugation stress, initial buckling stresses for several skin thicknesses, as well as the combined stress interaction curve are included.