On December 9, 1978 the neutral gas mass spectrometer aboard the NASA Pioneer Venus multiprobe bus has measured density, composition, and temperature of the Venus dayside thermosphere. There was no positive identification of argon down to the lowest measuring altitude of 130 km. For the altitude level of 135 km the following upper limits for the number densities of argon isotopes were derived: n(36Ar) <1.3¿106 cm-3 and n(40Ar) <2.8¿106 cm-3. From our upper atmosphere observations we infer for the troposphere of Venus the following upper limits for the mixing ratios: n(36Ar)/total number density <9¿10-6 and n(40Ar)/total number density <20¿10-6.