Interpretation of tracer tests is commonly based on the analysis of flow through a porous medium. In geothermal reservoirs however the principal permeability arises from fractures, and a porous medium approach is not applicable. The dispersion of tracer material flowing in a fracture is shown to be dominated by molecular diffusion across the fracture - a mechanism known as Taylor dispersion. For typical values of reservoir parameters, this transverse diffusion will obliterate any transverse tracer concentration gradients caused by the velocity profile across the fracture. The effective dispersivity is found to be a function of fracture aperture, mean flow velocity and molecular diffusivity, and is orders of magnitude greater than for porous medium flow. Typically Peclet numbers are anticipated to lie between orders 1 and 100, and field results confirm this range.