Single borehole pulse injection tests were carried out on single natural fractures in granitic rocks at a depth of 80 to 417 metres to determine the hydraulic aperture. Pressure pulses were created by injecting a small volume of water into the fracture using a compressed-air driven downhole valve. The pulses were of short duration with a fast pressure decay and apertures as small as a few microns could be measured. The field tests were modelled with a finite element program which simulates coupled fluid flow and mechanical deformation in both the fracture and the ambient rock matrix. The results of modelling show that if fracture flow dominates, then the storativity is dominated by the mechanical deformation of the fracture. For the short duration pulse tests used in this study, the deformation of the fracture and thus the storativity is restricted by the stiffness of the ambient rock. The results of the modelling indicated that the fracture storativity is close to 3.5 x 108. Field tests in hard rocks can therefore be evaluated with conventional curve matching methods using an assumed storativity to determine flow dimension and transmissivity. The known storativity makes the determination of hydraulic aperture of the fracture less subjective.