FM‐9 (the commercial name of an ICI product) in organic solvents or poly(methacrylic acid) (PMMA) in aqueous solution exhibit strong time‐dependent shear‐thickening behavior. The induction time needed for the onset of the thickening depends markedly on shear rate, solvent used, concentration, and molecular weight. Slightly altering the nature of the solvent profoundly affects the behavior. The effects are similar to those obtained by changing the concentration or molecular weight. The nature of thickening behavior is not well understood. The effect will be illustrated (using FM‐9 solutions) as a function of shear rate, concentration, and temperature. Defining a critical shear rate &ggr;˙cas that at which the induction time for thickening becomes so short as to be immeasurable within the resolution of the viscometer, &ggr;˙cfor this polymer is found to be inversely proportional to the zero shear‐rate viscosity of the solution for the temperature and concentrations studied.