An 8:1 multiplexer (MUX) and a 1:8 demultiplexer (DMUX) implemented with AlGaAs/GaAs heterojunction bipolar transistors are described. The circuits were designed for lightwave communications, and were demonstrated to operate at data rates above 6 Gb/s. These are among the fastest 8-b MUX-DMUX circuits ever reported. Each contains about 600 transistors and consumes about 1.5 W. The pair provides features such as resettable timing, data framing, and clock recovery circuitry, and a built-in decision circuit on the DMUX. ECL-compatible I/O signals are available. The circuits were implemented with bi-level CML logic and require a — 5.2-V power supply and a + 1-V bias for ECL compatibility.