Abstract <p>Dental anomalies of the lower and upper jaws caused by the proliferation of the apical parts of the molars were found in water voles from a laboratory colony. Such a molar phenotype is observed in several representatives of the subfamily Arvicolinae with constant growth of cheek teeth. In the water vole, apical elongation of molars of the upper jaw was found in 48.4% of individuals, and of the lower jaw in 53% of individuals (<i>n</i> = 589). The upper molars penetrate the cranial cavity of the animals, while the lower molars form bone growths on the buccal side of the jaw. Elongation of the molars on the lower and upper jaws occurs interrelatedly, as evidenced by the positive correlation between these features (<i>r</i> = 0.35). Ingrowth of molars into the cranial cavity is associated with the sex of the animals. In all age classes, the proportion of females with molar invasion into the skull is higher than that of males. The frequency of detection of dental anomalies is the highest at the age of 1–2.5 years. Positive correlations between the presence of a dental anomaly in siblings or in parents and offspring indicate the hereditary determination of the traits. The growth of the apical parts of the molars on the upper or lower jaw is associated with a decrease in the reproductive performance of females and does not affect the reproductive ability of males.</p>