Calcium oxalate (CaOx) is one of the common causes of kidney stones and accounts for 40 to 50% of all uroliths in cats. Oxalobacter formigenes, an oxalate-degrading intestinal microbiota, has been hypothesized to play a protective role against CaOx urolithiasis due to its capability to degrade oxalate. This study was designed to reveal the association between biomass colonization of O. formigenes and clinical occurrence of CaOx urolithiasis in household tomcats. Fifteen tomcats were allocated into three groups (healthy control (n = 5), static chronic kidney disease (static CKD) (n = 4), and progressive CKD (n = 6)) based on diagnosis of CaOx urolithiasis and disease progression. Fecal samples were collected from all tomcats, genomic DNA was extracted, and oxc, a gene specific for O. formigenes, was quantified using real-time PCR. Additionally, the clinical association between blood serum urea …