While examining the literature relating to the viscosity of aqueous solutions, the writer noticed some measurements obtained in connection with the viscosity of aqueous urea which appeared to warrant a reinvestigation of this substance. The relative viscosity of aqueous urea was accordingly measured at 18°C and 25°C over a range of concentration from 0.005 to 1.876 mol/liter. Fluidity— volume‐concentration curves exhibit negative curvature, evidencing that urea behaves like an electrolyte in that it decreases the fluidity of water less than would be expected on the basis of additive fluidities. It behaves like a non‐electrolyte in that it exhibits no stiffening effect; i.e., the value of the Falkenhagen constantAis zero. A table is included, giving a comparison of the observed and calculated values of the Falkenhagen constantAfor 11 electrolytes.