It has been suggested that isothermal flow-induced crystallization is linked jointly to shear strain and strain-rate. By combining continuum mechanics methods and experimental results, an invariant general strain-rate-strain model for some crystallizing polymers, applicable to all flows, is constructed. Examples show the great differences of response in shear and elongation; at a given rate elongation is the more powerful promoter of crystallinity. (C) 2009 Elsevier Ltd. All rights reserved.