The aim of the present study was to identify bacteria that may contribute to the onset of metabolic dysfunctions. We isolated and identified a candidate bacterium belonging to Lachnospiraceae (strain AJ110941) in the feces of hyperglycemic obese mice. The colonization of germ-free ob/ob mice by AJ110941 induced significant increases in fasting blood glucose levels as well as liver and mesenteric adipose tissue weights, and decreases in plasma insulin levels and HOMA-β values. These results indicated that the specific gut commensal bacterium AJ110941 influenced the development of obesity and diabetes in ob/ob mice with genetic susceptibility for obesity.