Abstract: Here we determine the individual age of a basal mosasaur (subfamily Halisaurinae) using zygapophyseal skeletochronology. This study is based on the cervical vertebra from the Late Cretaceous Beloe Ozero locality (Saratov Region). By of counting the zygapophyseal growth rings on the right prezygapophysis, it has been revealed that the age of the animal to which this vertebra belonged was at least 11 years. The absence of drastic reduction in the distance between the zygapophyseal growth rings is indicative of the fact that in the first 11 years of life, until the moment of death, the animal grew rapidly and evenly and reached a length of about 6 meters.