Previous theoretical discussion of flow and transport in a single fracture has led to a difference of opinion as to whether the aperture estimated from a tracer test need always be less than (or greater than) the aperture estimated from a concurrent hydraulic test. Interpretation of previously published field results has, to our knowledge, not provided a defensible example where the ratio of these two aperture estimates has varied from less than 1.0 to greater than 1.0 in the same fracture using the same analysis techniques. In the present paper, field experiments involving concurrent hydraulic and tracer tests have been run in two different directions within an isolated fracture. Aperture estimates were derived from both the hydraulic and tracer tests. The ratio of the aperture estimated from the tracer test to the aperture estimated from the hydraulic test is shown to be greater than 1.0 in the first experiment and less than 1.0 in the second experiment. Despite noise in the hydraulic test of the first experiment, it is argued that our interpretation is reasonable. As such, this represents the first set of experimental results known to us in which a defensible interpretation leads to the conclusion that the aperture ratio varies from greater than 1.0 to less than 1.0 within the same fracture. ¿ American Geophysical Union 1993