Abstract: Load carrier mountable above a bed structure of a vehicle having an upper surface and an internal bed rail system is disclosed. The load carrier broadly includes an upright base support member, an internal rail clamping assembly, and a clamp adjustment assembly. The upright base support member rests upon the upper surface of the side wall of the bed structure and connects with the internal rail clamping assembly. The upright base support member has a first clamp surface for asserting a clamping force upon the upper surface of the side wall of the bed structure. The rail clamping assembly is releasably securable to the internal bed rail system and includes a second clamp surface for asserting a clamping force upon the internal bed rail system. The clamp adjustment assembly positionally adjusts the first clamping surface with respect to the second clamping surface between clamped and unclamped positions.