Concrete beams ( 150 x 150 mm section ) containing steel or glass fibre reinforced polymer ( GFRP ) reinforcement were supported over a span of 1570 mm, and the load and deflection were measured as the beams were incrementally loaded in mid-span. The GFRP-reinforced concrete deflected significantly more than the steel-reinforced beam at similar loadings, and also exhibited more cracking. Deflection decreased with increasing concrete compressive strength, the effect the more significant with the GFRP-reinforced beams. This was attributed to increased tension-stiffening and it is proposed that the effect should not be ignored in GFRP beams. The GFRP-reinforced beams had the higher ultimate strength. The predictions of a spreadsheet developed to calculate theoretical deflections were in reasonable agreement with the experimental observations except for GFRP-reinforced beams with low concrete compressive strength. 3 refs.