Following development of a filament-stretching extensional rheometer at Monash University, similar rheometers have been designed and built in other laboratories. To help validate the basic technique, a collaborative program was undertaken to compare results from several instruments. First, three test fluids prepared at the University of California at Berkeley were characterized in steady and transient shear flows there and at the Massachusetts Institute of Technology (M.I.T.), and then tested in extensional rheometers at M.I.T., Monash and the University of Toronto. Each fluid is a constant-viscosity solution of narrow-molecuIar-weight-distribution polystyrene dissolved in oligomeric polystyrene. The solute molecular weights are 2.0, 6.5, and 20 million g/mol, and the polymer concentration in each fluid is 0.05 wt.%. From linear viscoelastic measurements, the Zimm relaxation times of the fluids are found to be 3.7, 31, and 150 s, respectively. The scaling of relaxation times with molecular weight indicates better-than-theta solvent quality, a finding consistent with independent intrinsic viscometry measurements of equilibrium coil size. Each fluid was tested in the three filament stretching rheometers at similar Deborah numbers. Despite variations in instrument design and the general difficulty of the technique, transient Trouton ratios measured in the three instruments are shown to agree quantitatively. (C) 2001 The Society of Rheology.