The behavior of two 55 ft. long prestressed, composite steel beamconcrete slab bridge units was studied. The type of unit tested is currently used in county road bridge construction, where the use of prefabricated units is especially economical. In primary test phases, the first unit was subjected to 3 years of sustained loading, over 2,000,000 cycles of fatigue loading and statically loaded to failure. The second unit underwent 500,000 cycles of fatigue loading and was statically loaded to its yield level. In supplementary test phases, pushout-type specimens with channel and stud shear connectors, identical to those in the bridge units, were studied to determine the difference between the two connector types under sustained .;Ird ultimate loading conditions. In addition, transverse slab strength tests were performed at six locations on the first unit, and on six similar, simply supported, control slabs. The transverse slab strength tests were performed to verify that arching action occurs in the bridge slab. The presence of arching action in the bridge slab changed the mode of slab failure from a relatively ductile flexural failure, to a sudden punching failure at a much higher concentrated load. Test results were compared to theoretical predictions and AASHTO Specification limitations. It was found that the behavior of the unit was reasonably predictable, and that with a minor connection detail change, the prestressed, composite steel beam design concept is suitable for county road bridge use.