Basic principles underlying the methodology of the procedure of entering and deleting biological species in regional Red Data Books are shown. With Natrix tessellate as an example, an algorithm is demonstrated to exclude a species from the list of rare and endangered animals needing special protection measures. As a result of our longterm field studies of the N. tessellata populations, a positive trend in the abundance change in its population and in the spectrum of biotopical correspondence and geographical distribution within the Saratov region has been established.
