Peritoneal infections caused by Mesocestoides spp. are rare in dogs and cats. Little data exist on the role of abdominal ultrasonography for diagnosis and therapy management of the disease. We describe the ultrasonographic features of peritoneal cestodiasis in a dog and in a cat. In the dog, abdominal ultrasound allowed both a presumptive diagnosis and the collection of tissue samples to confirm peritoneal larval infection. Ultrasound was also very useful for therapy management. In the second patient the ultrasonographic features of tetrathyridial infection in a cat in which the parasite was observed as an incidental finding during ovariohysterectomy are described.