In this paper we study the hierarchic control problem for a linear system of a population dynamics model with unknown birth rate. Using the notion of low regret control and an observability inequality of Carleman type, we show that there exist two controls such that, the first control called follower solves an optimal control problem which consist in bringing the state of the linear system to desired state, and the second one named leader is supposed to lead the population to extinction at final time. Mathematics Subject Classification. 49J20,92D25,93B05; 93C41.