Equations have been derived on the basis of general ratios of the numbers of different age groups of animals that allow to estimate the mortality coefficient in populations with stationary age structure and intensity of mortality that does not depend on age at their continuous and discontinuous replenishing by frequency of occurrence of individuals of the maximal age. The proposed method of estimation of mortality has been tested on models with different numbers of animals and age classes. The equation gives a non-biased estimate of mortality coefficient for populations with discontinuous replenishing with young individuals at very small sample sizes (less than 10 individuals).