Abstract Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal forms of cancer, with an incidence rate almost equal to its mortality rate. It has few strongly predictive risk factors, either genetic or environmental, it most often produces symptoms at advanced, inoperable stages, and it is resistant to both generic and targeted therapies. PDAC patients have had little benefit from the advances in early detection, prevention and treatment that have improved care for other common cancers. Intriguingly, however, recent evidence suggests that the microbial community of the gastrointestinal tract – the gut microbiota – represents a potential source of both biomarkers and therapeutic targets in PDAC. Published studies, and our own preliminary work, indicate that the microbiota is required for efficient pancreatic tumor initiation, although the underlying species and molecular mechanisms remain unknown. In addition, the microbiota of human PDAC patients is distinct from healthy controls, and early microbial alterations represent risk factors for later cancer development. Again, the mechanistic basis for this relationship is unknown, as it is a challenge to establish cause-and-effect relationships through observational studies, or through the use of conventional mouse models that lack control over the timing of tumor-initiating events. We hypothesize that the microbiota harbors a diverse array of cancer-promoting and -inhibiting species, acting at multiple times and through multiple mechanisms including modulation of immune responses, and that inter-individual heterogeneity in these species contributes to PDAC risk and disease outcome. To test this hypothesis, and to identify components of the microbiota as predictive markers of disease and targets for prevention and treatment, we propose to establish an experimental platform for functional analysis of the microbiota, based on a temporally-inducible mouse model of endogenous PDAC initiation and progression. We will use orthogonal analyses of time-specific and antibiotic-specific microbial ablation to dissect the roles of the endogenous mouse microbiota (Aim 1), and establish mice with “humanized” microbiota from individual PDAC patients and healthy controls to characterize the effects of disease-specific microbial communities on tumorigenesis and anti-tumor immunity (Aim 2). Our work will directly identify relevant members of the microbiota that contribute functionally to pancreatic cancer risk, and provide the knowledge base to develop microbial biomarkers and therapies to mitigate cancer progression.