Addition of particles to a viscoelastic suspension dramatically alters the properties of the mixture, particularly when it is sheared or otherwise processed. Shear-induced stretching of the polymers results in elastic stress that causes a substantial increase in measured viscosity with increasing shear, and an attractive interaction between particles, leading to their chaining. At even higher shear rates, the flow becomes unstable, even in the absence of particles. This instability makes it very difficult to determine the properties of a particle suspension. Here, we use a fully immersed parallel plate geometry to measure the high-shear-rate behavior of a suspension of particles in a viscoelastic fluid. We find an unexpected separation of the particles within the suspension resulting in the formation of a layer of particles in the center of the cell. Remarkably, monodisperse particles form a crystalline layer which dramatically alters the shear instability. By combining measurements of the velocity field and torque fluctuations, we show that this solid layer disrupts the flow instability and introduces a single-frequency component to the torque fluctuations that reflects a dominant velocity pattern in the flow. These results highlight the interplay between particles and a suspending viscoelastic fluid at very high shear rates. Suspensions of solid particles in a fluid are widely encountered in all ranges of technology both as products and as precursor materials in manufacturing and materials preparation. They exhibit a wide range of properties depending on the particle concentration or volume fraction, ϕ , and on the nature of the suspending fluid. When the suspending fluid is Newtonian, the viscosity of the suspension increases slowly with increasing ϕ at low volume fractions, and then diverges as ϕ approaches the maximum volume fraction of randomly packed solid spheres ( 1– 3). As the shear rate increases, higher-volume-fraction suspensions exhibit slight shear thinning with the viscosity decreasing with increasing shear rate, while at very high shear rates, they undergo a sudden, dramatic shear thickening, becoming almost solid-like due to increased interparticle collisions that result in frictional interaction between the particles ( 4– 6). The behavior of a suspension is significantly different if the suspending fluid is viscoelastic ( 7– 11). At low ϕ and low shear rates, the particles follow streamlines, and the behavior of the suspension is similar to that of the suspending fluid ( 12, 13). As the shear rate increases, however, the large polymer molecules become increasingly stretched and cannot fully relax, resulting in a stress normal to the streamline, causing particles to cross streamlines ( 14– 18), particle chaining ( 19– 25) and shear thickening ( 26– 29). At high shear rates, the stretched polymer exerts yet larger elastic stresses; even in the absence of particles, these stresses dominate over viscous stress and can destabilize the flow field, resulting in an elastic instability and so lead to the development of a secondary chaotic flow ( 30– 36). This instability makes it challenging to quantify the behavior of a viscoelastic suspension of particles at high shear rates; as a result, only low shear-rate regimes, where the instability is avoided, have been investigated, and the behavior of particles at high shear rates remains unexplored. However, the interplay between fluid elasticity and particles at high shear rates is important for a wide range of industrial processes, such as hydraulic fracturing ( 37– 39) and polymer composite processing ( 12, 40), as well as natural phenomena like biofluid flow ( 41– 44).