A 14-month-old female dog, Gos d'Atura Catala was presented for a 7-month-history of reoccurring urinary tract infection. Using sonography, a focal multilobulated thickening of the urinary bladder wall was discovered. The solid mass was arising from the area of the ureteral papllDae and bulging into the lumen of the bladder neck. The wall of the urethra was uniformly thickened. These findings were not considered typical for a generalized urinary tract infection but more indicative of local severe inflammation, neoplasia or hyperplasia in the area of the ureteral openings. The thickening of the urethra was suggestive of urethritis or neoplastic infiltration. Signs of metastasis were not detected on the thoracic radiographs or in the remainder of the abdominal ultrasound examination. A surgical excision of the multilobulated mass was performed and histologic examination was conducted. A fibrosarcoma in the lamina propria of the urinary bladder wall was diagnosed. Because of reoccurence of hematuria and unresponsiveness to therapy the dog was euthanized. Postmortem examination confirmed the diagnosis of fiborsarcoma in the urinary bladder. Additionally, neoplastic infiltration of the urethral wall and metastasis in the lungs and liver were detected histopathologically.