This paper reports on the composite action developed between pultruded FRP bridge deck systems and the supporting main girders for road bridges. Presented are requirements that an FRP deck must satisfy in order to function as part of the upper chord of the main bridge girders. It is shown that the deck must exhibit adequate in- plane compression and shear capacity and stiffness in the longitudinal direction of the bridge. Laboratory experiments using an existing FRP deck system with trapezoidal cell geometry are presented and their results analyzed to establish in-plane system properties for bridge design and dimensioning. These system properties include the effects of the material properties, the cross-sectional geometry and the adhesives used to bond the individual pultruded shapes to form the deck. The influences of the cell geometry (trapezoidal or triangular), the fiber architecture and the adhesive used for the deck joints (epoxy or polyurethane) on the system properties are discussed. © 2004 Elsevier Ltd.