Small intestinal submucosa (SIS) has been studied as a potential biomaterial for use in tissue engineering applications. Extracted from the mammalian small bowel, it consists of two collagen fiber populations at ∼±30° from the longitudinal axis. We have previously investigated the biaxial mechanical properties of SIS (Sacks and Gloeckner, 1998), which were comparable to other bioprosthetic biomaterials. Our long-term goal is to develop structural constitutive models of implantable biomaterials made from SIS and other acellular materials. These models can aid in determining how well a biomaterial will perform in the virgin and remodeled states. Structural models require quantitative morphologic information, especially fiber structure and kinematics. In this study, we examined the change in the fiber kinematics at different states of biaxial stretch and developed an initial structural constitutive model for SIS.