Four cases of bacterial cystitis in female guinea pigs are described. The diagnosis was based on urinary examination, including bacterial cultures with sensitivity testing, and ultrasonographic examination. Different bacteria were isolated, with the most common being Escherichia coli, often having multiple antibiotic resistances. All animals were treated with antibiotics following sensitivity testing, analgesics and bladder flushing. Although the clinical and general condition of the animals improved, all guinea pigs had repeated acute relapses of the condition. In all cases, the urine contained coagulated blood. During later episodes, it also contained tissue debris. The ultrasonographic examination showed severe exfoliation of the bladder wall and was considered to be a type of necrotising cystitis. Two animals also developed suspected encrusting cystitis classified by a hyperechogenic covering on parts of the bladder wall. To the author's knowledge, this is the first report of these unique forms of cystitis in guinea pigs.