The recently suggested solar wind quasi-invariant [QI≡(B2/8&pgr;)/(&rgr;v2/2)> is a combination of magnetic field strength B, plasma density &rgr; and solar wind speed v. Measured in the solar wind near the Earth, the median value of QI was shown to be a reliable index of solar activity for the 28 year period holding a linear relation with sunspot numbers (SSN) with a high correlation coefficient (cc) of 0.98 [Osherovich, Fainberg and Stone, 1999>. Here, using hourly data from the Pioneer Venus Orbiter for the period from 1978--1988, we demonstrate that QI near Venus also closely follow SSN with a correlation coefficient of 0.94 for the yearly mean values. We found that the calibration coefficient between QIVenus and QIEarth is slightly larger than unity, namely, QIVenus=a*⋅QIEarth where a*=1.134¿0.001. We suggest that, for any point in the heliosphere, QI can be viewed as a useful measure of solar activity. ¿ 2001 American Geophysical Union