Stress levels in elongational flow of suspensions of long fibers have been determined experimentally using a jet thrust apparatus. Stress levels for fibers with an axis ratio of 340 and a volume fraction of 0.001 exhibit stresses nearly an order of magnitude above values for a pure Newtonian fluid. A recent theory by Batchelor is shown to give the correct dependence of stress on axis ratio and concentration, but the predicted magnitude of stress is generally lower than the experimental values.