Polymineralic rocks display three end-member types of mechanical and microstructure behavior; (1) strong minerals form a load-bearing framework that contains spaces filled with weaker minerals; (2) two or more minerals with low relative strengths control bulk rheology and form elongate boudins; (3) one very weak mineral governs bulk rheology, while the stronger minerals form clasts. A concept of strain energy partitioning in polyphase aggregates allows the expression of the bulk strength of a rock in terms of its strain rate, temperature, and the volume proportions and rheological properties of its constituent minerals. Bimineralic strength versus composition relations predicted with this model are highly nonlinear and mimic the rheological behavior of experimentally deformed bimineralic aggregates. Quantitative analysis of microstructures in quartzo-feldspathic tectonites suggests that foliation development leads to a significant reduction in rock strength. This is due to decreased stress concentration within the deforming aggregate as matrix and clast grain size is reduced and the average spacing between strong minerals increases. ¿ American Geophysical Union 1990