Abstract. In this paper, we develop a criterion to calculate the multiplicity of a multiple focus for general predator-prey systems. As applications of this criterion, we calculate the largest multiplicity of a multiple focus in a predator-prey system with nonmonotonic functional response p(x) = x ax 2 +bx+1 studied by Zhu, Campbell, and Wolkowicz [SIAM J. Appl. Math., 63 (2002), pp. 636-682] and prove that the degenerate Hopf bifurcation is of codimension two. Furthermore, we show that there exist parameter values for which this system has a unique positive hyperbolic stable equilibrium and exactly two limit cycles, the inner one unstable and outer one stable. Numerical simulations for the existence of the two limit cycles bifurcated from the multiple focus are also given in support of the criterion.Key words. predator-prey, Liénard system, multiple focus, Hopf bifurcation, codimension two, limit cycles AMS subject classifications. Primary, 34C25, 92D25; Secondary, 58F14 DOI. 10.1137/0506234491. Introduction. The existence and number of limit cycles are important topics in the study of most applied mathematical models. Such study has made possible a better understanding of many real world oscillatory phenomena in nature [1,11,17]. For predator-prey systems, it is well known that the existence of limit cycles is related to the existence, stability, and bifurcation of a positive equilibrium. In a positively invariant region, if there exists a unique positive equilibrium which is unstable, then there must exist at least one limit cycle according to the theory of Poincaré-Bendixson. On the other hand, if the unique positive equilibrium of a predator-prey system is locally stable but not hyperbolic, there might be more than one limit cycle created via Hopf bifurcation(s). Numerical simulations of Hofbauer and So [8] indicated that this is indeed the case: there can exist at least two limit cycles for some two-dimensional predator-prey systems. It was proved by Zhu, Campbell, and Wolkowicz [26] that a predator-prey system with nonmonotonic functional response can undergo a degenerate Hopf bifurcation which produces two limit cycles, and the system can also have two limit cycles through a saddle node bifurcation of limit cycles. These two limit cycles can disappear through either supercritical Hopf or/and homoclinic bifurcations. Kuang [9] and Wrzosek [22] also observed that some predator-prey systems can even have more than two limit cycles. We shall point out that in all the models studied in [8,9,22] the death rate of the predator is nonlinear, but ours in [19,26] is linear.Hopf bifurcation theory is a powerful tool for studying the existence, number, and properties of the limit cycles in mathematical biology. However, the largest number