Guinea pigs (Cavia porcelles) are susceptible to formation of urinary tract calculi. Uroliths can be located anywhere in the urinary tract and are typically composed of calcium salts. A pet, female guinea pig, aged 5 years and weighing 0.5 kg was referred to Internal Medicine Department Clinics for stranguria and recurrent haematuria over 15 days. The guinea pig was emaciated, dehydrated and hypothermic, also demonstrated pain on caudal abdominal palpation and a small thickened bladder wall was discovered. Gastric dilatation and urinary calculus was detected with thoracoabdominal radiography. 2 days later death was learned and then it was brought for necropsy to Department of Pathology. Necropsy revealed calculus in the lumen of urinary bladder with the dimensions of 0.7x0.5 cm though no distinct changes were grossly evident in the urinary tract. The bladder was empty and the mucosa was moderately thickened suggestive of fibrosis and edema resulting from a possible chronic condition. Physical and chemical analysis of the calculus was done by laboratory. Calcium and oxalates are the main risk factors for stone formation, but the calculus of this guinea pig included calcium carbonate and oxalate in combination with other minerals; including struvite. To the authors' knowledge, it was the first guinea pig including urinary signs and calculus, presented in Turkey.