Clostridium difficile has been recognized as a significant cause of illness in neonatal pigs, but is not encountered in piglets older than 1 week of age. We hypothesized that this age related resistance is due to the increase in bacterial diversity (species and numbers of bacteria) in the intestinal tract that occurs as piglets grow and are exposed to more of their environment. Piglets were challenged with C. difficile at 2- 14 days of age and the microbial diversity of the cecal contents was determined. Half of the piglets that were challenged with C. difficile at 2 and 4 days of age showed signs of disease. The incidence of disease decreased with piglet age and none of the piglets challenged at >10 days of age showed any signs of disease. The bacterial populations of piglets also clustered by age so that the populations from piglets that were 2-4 days of age were more closely related to one another than to the populations in older piglets. This relatedness occurred across litters from 4 different sows and supports our hypothesis that the resistance to C. difficile disease in piglets greater than 1 week of age may be directly related to the diversity and complexity of the microbial community in intestinal tract.