The intestinal Oxalobacter Formigenes were isolated in 30 cases of urolithaiasis and in 45 controls. The biologic characters and morphology of the bacteria were also observed. The results showed that the colony counts in urolith group 9 (mean 10(3)/g. faeces) were significantly less than that of controls (mean 10(8)/g. faeces) (P < 0.001). It is believed that the lesser amount of oxalobacter formigenes in urolith was the important factor of the calcium oxalate calculi formation.