Stress-rupture and tensile tests were conducted at 816 deg C (1500 deg F) to determine the effect of matrix composition on the minimum fiber length to diameter ratio (critical aspect ratio) below which fibers in a tungsten fiber/copper-nickel alloy matrix composite could not be stressed to their ultimate load carrying capability. This study was Intended to simulate some of the conditions that might be encountered with materials combinations used in high-temperature composites. The critical aspect ratio for stress-rupture was found to be greater than for short-time tension, and it increased as the time to rupture increased. The Increase was relatively slight, and calculated fiber lengths for long service appear to be well within practical size limits for effective reinforcement and ease of fabrication of potential gas turbine components.