A low molecular weight bisurea in nonpolar solvents is shown to self-assemble by hydrogen bonding into two distinct high molecular weight structures. At low temperature and high concentration, the most stable structure is a thick cylindrical assembly, responsible for the very high viscosity of the solution. At higher temperature or lower concentration, the thick filaments disappear in favor of thinner filaments, leading to a lower viscosity. The reversible transition occurs over a temperature range of 5 degrees C only, showing that it is highly cooperative. The structural switch can also be triggered by changing the nature of the solvent or the composition in the case of a mixture of two bisureas. The high cooperativity and the tunability of this transition are useful for the design of responsive materials.