In 1969 s single lane road bridge was constructed in the Redesdale Forest using Lytag lightweight concrete prestressed inverted T beams with an in situ lightweight concrete deck slab. Four beams from the same production run were tested in the laboratory to determine their ultimate strength under static and repeated loading conditions and to establish the cracking behaviour and ultimate mode of failure. The test loading was considerably more severe than it would be for this type of beam in service, the cyclic load chosen being that load which produced a crack 0.2 mm wide. Under static loading final collapse was due to buckling of the simulated deck slab, followed by compression failure of the beam flange. Under cyclic loading equivalent to 60 percent of the ultimate strength, fatigue failure of the prestressing strands occured after nearly 300,000 cycles, following extensive cracking and fragmentation of the lightweight concrete in the tension zone. Cracking was much more extensive than on conventional concrete beams of similar design, with general fracturing of the aggregate particles