This paper analyses the extensibility under flexural loads of concretes with 28-day compressive strength levels up to 80 MPa. The major variables studied were loading rate, aggregate type, testing age and the environmental (moisture) conditions. It is shown that extensibility increases with concrete strength but its variation is smaller than that of concrete strength. It was verified that in high-strength concrete the extensibility is affected by loading rate. Extensibilities of 200 and 350 microstrains were measured under rapid and slow loading conditions, respectively. There are also differences in the stress-strain behaviour produced by changes in concrete microstructure (in this case due to drying shrinkage). The data presented are a significant contribution to predicting high-strength concrete extensibility.