In this paper a throughput and delay analysis of FDDI and FDDI-II is presented under mixed traffic. The case of voice/data transmission is considered, and the throughput and delay behavior of FDDI, under packetized voice transmission, and FDDI-II, under isochronous voice transmission, is examined. The performance of the two networks is then investigated under the transmission of voice, data, and video. Two transmission alternatives are considered. In the first, all three types of information are transmitted in packets over an FDDI network. In the second, the isochronous slots of FDDI-II carry the voice and video. The effect of various system parameters on the performance of both networks is examined, and their voice, video, and data performance are compared.