Commensal bacteria are critical for physiological functions in the gut, and dysbiosis in the gut may cause diseases. In this article, we report that mice deficient in cathelin-related antimicrobial peptide (CRAMP) were defective in the development of colon mucosa and highly sensitive to dextran sulfate sodium (DSS)-elicited colitis, as well as azoxymethane-mediated carcinogenesis. Pretreatment of CRAMP-/-mice with antibiotics markedly reduced the severity of DSS-induced colitis, suggesting CRAMP as a limiting factor on dysbiosis in the colon. This was supported by observations that wild-type (WT) mice cohoused with CRAMP-/-mice became highly sensitive to DSS-induced colitis, and the composition of fecal microbiota was skewed by CRAMP deficiency. In particular, several bacterial species that are typically found in oral microbiota, such asMogibacterium neglectum,Desulfovibrio piger, andDesulfomicrobium orale, were increased in feces of CRAMP-/-mice and were transferred to WT mice during cohousing. When littermates of CRAMP+/-parents were examined, the composition of the fecal microbiota of WT pups and heterozygous parents was similar. In contrast, although the difference in fecal microbiota between CRAMP-/-and WT pups was small early on after weaning and single mouse housing, there was an increasing divergence with prolonged single housing. These results indicate that CRAMP is critical in maintaining colon microbiota balance and supports mucosal homeostasis, anti-inflammatory responses, and protection from carcinogenesis.