This study considers a large‐scale flow maintained by a small‐scale periodic force field. If the field is essentially anisotropic, the system of small‐scale eddies thereby generated turns out to be unstable to long‐wave disturbances, i.e., the effective viscosity of the corresponding large‐scale flow is negative. If the applied field is sufficiently isotropic, the long‐wave instability disappears so that the effective viscosity is positive.