The collision shifts of two argon lines, λ 7067 A and λ 6965 A, have been measured, the lines being excited in a D.C. glow discharge in pure argon. The lines were examined by means of a Fabry–Perot interferometer crossed with a Hilger prism spectrograph. The interference ring diameters were measured for a variety of argon pressures and discharge currents. It is shown that the measured shifts were due to collisions between two argon atoms, and not to the influence of charged particles. The shift for λ 7067 is − 0.61 ± 0.05 × 10 −20 cm −1 per atom per cc of argon, and for λ 6965 is − 0.65 ± 0.05 × 10 −20 cm −1 per atom per cc of argon, where the negative sign indicates a red shift. These results are shown to imply that the short range repulsive forces between argon atoms can be of little importance in producing the shifts, in contrast to the case of the calcium line λ 4227 A perturbed by helium.