The stability of a multipass reactor having series coolant flow through the passes was considered. The investigation was predicated on the assumption that the effects of the temperature coefficient of reactivity of each pass can be superimposed. An analytical model was derived based on constant coolant flow and constant core inlet temperature. By making certain simplifying assumptions, a criteria was established which predicts the stability of the system as a function of the temperature coefficients of reactivity. It has been shown that a negative temperature coefficient in each pass is neither a necessary nor a sufficient condition for stable operation.