Dental growth lines are often counted for age determination in feral animals that are living in the wild and are subject to seasonal variations. To assess whether or not this technique could also be used in domestic animals, the cementum growth lines of the canine and second premolar teeth were determined in 4 red foxes (Vulpes vulpes) living in the natural habitat of Flanders, and in 7 domestic dogs (Canis familiaris) that were kept in housing conditions. Dates of birth of all animals were registered. In the foxes a good correlation was found between the age of the animals and the number of cementum growth lines. In contrast, in domestic dogs the number of cementum lines was variable and could hardly be determined because the difference between primary and secondary lines was often uncertain. It was concluded that the count of cementum growth lines cannot be used for age assessment of dogs that are living in domestic conditions devoid of seasonal variations in food accessibility and weather conditions.