In future broadband ISDN both large-capacity transmission lines and high throughput for handling a large number of high-speed signals, such as full-motion video, will be required for transmission/switching systems.1 High-throughput interconnection techniques for inter- and intra-frames, units, boards, and LSI chips are expected for realizing these systems. In particular, optical parallel-transmission techniques2 are especially important in high-throughput interconnection because electrical parallel-transmission techniques limit the transmission capacity, transmission distance, and flexibility of system installation. In this paper we report on gigabit-per-second optical parallel trans- mitter/receiver modules with GaAs MESFET LSI chips for overcoming the electrical limitations.