A data distibution evaluation system (DDES) has been developed for use by NASA Langley Research Center to evaluate fiber optic data bus systems. The background leading to the system, a general description of the system, and some plans for use of the system are given. The DDES is a three station system with each station containing a microprocessor and a micoprogrammable bus control interface unit (BCIU). The systems are interconnected by both dual twisted pair buses (MIL-STD-1553B) and by dual 55 micron fiber optic data buses (proposed DOD-STD-XXFO). Use of writeable control store for the microcode of the BCIU allows varying protocols to be evaluated using only software changes. Use of removable fiber optic transmitter/receivers allows evaluation of different modulation techniques and transmission media by making front-end hardware changes. The three DDES stations are used alternately as system controller, remote terminal, and bus monitor, with automatic rotation of the functions among the stations. As delivered, the fiber optic portion of the system is connected using a transmissive star coupler and uses bi-level manchester encoding. Planned experiments include modification of the hardware to compare the four modulation schemes proposed for MIL-STD-XXFO, and evaluation of a four port wavelength division multiplexing (WDM) system currently under development. An overview of MIL-STD-XXFO and of the wavelength division multiplexing system is given along with the planned use of the DDES to evaluate their performance.