We consider the flow of a fluid with a modified Bautista-Manero rheology injected radially into a porous medium or unidirectionally into a widening fracture. Linear stability analysis of the flows demonstrates that viscous fingering instabilities are possible, supported essentially by the thixotropic nature of the fluid: the growth rate of unstable perturbations scales approximately with the relaxation rate lambda(-1) of the fluid, while their spatial scale is set in the radial case by (Q lambda)(1/2) where is the areal injection rate, and in the fracture case by a 'relaxation lengthscale' which cannot be deduced by purely dimensional reasoning. It appears likely that the same instability mechanism can occur in other shear-thinning thixotropic fluids. (c) 2006 Elsevier B.V. All rights reserved.