Results of a 3-year-long investigation of durability of one type of glass fiber reinforced vinylester composite reinforcement bar are summarized. Bars were cast in concrete beams and subjected to simultaneous sustained load and exposure to one of four different environments: ambient indoor laboratory, natural outdoor weathering in central Pennsylvania, high-alkaline aqueous solution at 60, and alternating -17 dry freeze and room-temperature water immersion. The conditioned beams were tested to determine crack width in the concrete, local bond-slip behavior of the bars, and tensile stress-strain behavior of bars extracted from the beams. Over time, crack widths increased by up to 75% while local ultimate bond strength in the anchorage zone remained essentially constant or increased. Tensile strength decreased by as much as 25% in the high moisture environments and was essentially constant in the indoor and outdoor environments. These results suggest promising durability characteristics of GFRP bars under realistic service conditions.