A study was made of the viscosities of three ethylene-propylene copolymers in a naphthenic base oil. Two of the copolymers contained, respectively, 60 and 70 mol% ethylene and were amorphous while the third had 80 mol% ethylene and was partially crystalline. At room temperature, the solutions of both amorphous and partially crystalline copolymers were Newtonian. At low temperatures, the solution of the partially crystalline copolymer became non-Newtonian and gave viscosity values which were much lower than expected on the basis of molecular weight. These results are interpreted in terms of formation of ordered domains or aggregates in the solutions of the partially crystalline copolymers at low temperatures.