AbstractSince the beginning of the 20th century, the number of saigas (Saiga tatarica tatarica L., 1766) sharply decreased twice as a result of human activity. After the first decrease, all four saiga populations (the Northwest Pre-Caspian, Ustyurt, Volga–Ural, and Betpakdala) recovered, and they numbered more than 1 million heads by the 1970–80s. However, after the second decrease (since the beginning of the 21st century), an increase in the number of these ungulates has been noted only for two populations, the Volga–Ural and Betpakdala. The goal of the study is to analyze the biological parameters of the four populations and to identify the differences that allowed the saigas inhabiting the Volga–Ural interfluve and Betpakdala to overcome population depression and to recover their numbers. At the end of the 20th century, the economic situation in the countries inhabited by saigas promoted poaching among the population, which led to a sharp decrease in the total number of these ungulates down to 50 000. The area of the main range decreased by 50 times for the Northwest Pre-Caspian saiga population and by 2.4–5 times for the other three populations. The distance of seasonal migrations, as well as animal involvement, also decreased. The saiga population of the Northwest Pre-Caspian has become practically sedentary. The low-number periods are characterized by a decrease in herds of different sizes found throughout the year; herds with less than 50 animals were registered in 56–100% of cases. In the early 2000s, the main biological parameters characterizing the four saiga populations were comparable. Moreover, the proportion of adult males and the yield of young animals per female in July–August were higher in the population of the Northwest Pre-Caspian in comparison to the other three (11.5 ± 1.9% and 0.81 ± 0.18 ind. vs. 8.7 ± 3.8% and 0.48 ± 0.40 ind., respectively). The increase in the Volga–Ural and Betpakdala populations is mainly due to their effective protection and the education of the local people with the support of the Kazakhstan government and international foundations, which significantly reduce the poaching pressure.