A frame-invariant constitutive model is developed to describe the changes in droplet anisotropy and stress in a dilute blend of Newtonian droplets in a Newtonian matrix after imposition of a large step strain. The model, based on the Tomotika theory [Proc. R. Soc. London, Ser. A 150 (1935) 322], describes the evolution of components of the anisotropy tensor from values consistent with a cylindrical shape with undulations to values consistent with a series of fragmented spherical droplets. This model is compared to experimental measurements of droplet shape anisotropy for single droplets that were deformed to various degrees and then allowed to relax. Here, the droplet shape anisotropy is defined using the Doi-Ohta anisotropy tensor [J. Chem. Phys. 95 (1991) 1242], which is obtained by integrating over the droplet interfacial position as determined by microscopic image analysis. For moderately extended droplets, the anisotropy is affected by the ends of the droplet, and thus does not agree with breakup model predictions based on the Tomotika theory, which neglects contributions from the ends of the droplet. When droplet end-effects are subtracted, and only the effects of undulations in the droplet center remain, good agreement between theory and experiment is achieved. Data at various strains also superimpose on a single breakup line when end-effects are neglected. (C) 2003 Elsevier B.V. All rights reserved.