NASA Glenn Research Center and the Wallops Flight Facility jointly conducted a PEM fuel cell power system development effort for high altitude balloon applications. This was the first phase of NASA efforts to offer higher balloon payload power levels with extended duration mission capabilities for atmospheric science missions. At present, lead-acid batteries typically supply about 100 watts of power to the balloon payload for approximately 8 hours duration. The H2-O2 PEM fuel cell demonstration system developed for this effort can supply at least 200 watts for 48 hours duration. The system was designed and fabricated, then tested in ambient ground environments as well as in a thermal vacuum chamber to simulate operation at 75 kft. altitude. Initially, this program was planned to culminate with a demonstration flight test but no flight has been scheduled, thus far.A safe, low cost, simplified design was targeted for this program as the objectives were to demonstrate the technical feasibility of the fuel cell system and to develop the design basis for an optimized power system. It was therefore desirable to collect and log as much operational data as possible since those data would be used for the design of the advanced system. Thermal management posed the major operational design challenge since the fuel cell operating temperature was constrained to be between 60-100°C, the fuel cell product water was maintained above 0°C, and the data logger and control electronics were required to be maintained above 10°C. For operational simplicity, a completely passive power system was designed to eliminate the need for ancillary equipment such as heat exchangers, humidifiers, and reactant feed pumps. Aside from the fuel cell stack itself, all system components were available off-the-shelf and where possible, excess components from other NASA programs were used. In this paper, details of the design, fabrication, and testing phases of the development effort will be explained and operational data will be presented.