A clinical case of thymoma in 6 month old, male, cavalier King Charles spaniel was described. The patient was brought to the Small Animal Clinic of Veterinary faculty at Trakia University, Stara Zagora in order to be evaluated by means of electroretinography (ERG) because of vision loss and difficulties in orientation in unfamiliar surroundings. Physical examination did not reveal any disturbances in vital parameters neither in retinal function. As complications occurred during recovery from general anaesthesia more detailed investigations were performed and the diagnosis thymoma was made. It was confirmed pathohistologically. Thymoma is an extremely rare tumor in dogs, especially in cavalier King Charles spaniels. The unusual manifestation of the disease as well as its very rare distribution among dogs was the reason to give a problem consideration.