This paper reports experiments on particle migration in viscoelastic fluids used in hydraulic fracturing. It is found that particle migration in such fluids is controlled by the elastic properties of the suspending fluid and the shear rate gradient. In fluids with low but measurable normal stresses and dominant shear-thinning properties, particles migrate to regions of lower shear rate. Migration is fast initially, but slows down rapidly over a short distance. For these fluids the bulk migration velocity correlates with the product of the Weissenberg number and the mean shear rate gradient. In contrast, highly elastic fluids with relaxation times well above one second and shear-thickening properties at low shear rates, flow with a central plug region or slip at the wall, producing little or no migration.