Summary: This paper deals with the question of uniqueness of limit cycles in predator-prey systems of Gause type. By utilizing several transformations, these systems are reduced to a generalized Lienard system as discussed by

*L. A. Cherkas* and

*L. I. Zhilevich* [Differ. Uravn. 6, 1170-1178 (1970;

Zbl 0213.364)] and by

*Z. Zhang* [Appl. Anal. 23, 63-76 (1986;

Zbl 0582.34038)]. As a consequence, criteria for the uniqueness of limit cycles are derived, which include results of

*K.-S. Cheng* [SIAM J. Math. Anal. 12, 541-548 (1981;

Zbl 0471.92021)] and are related to results of L.-P. Liou and K.-S. Cheng (to appear). Several examples are given to illustrate our results.