This paper reports astronomical seeing measurements made at four sites in the western United States during 1991-92. This study was conducted as part of the site selection process for the Navy Prototype Optical Interferometer. An extensive series of observations were made at Chews Ridge and Anderson Ridge, CA (~36,000 equivalent FWHM seeing estimates). Observations made from Chews Ridge over 49 nights yield an average FWHM seeing value of ~1.1", while those made from Anderson Ridge over 34 nights yield an average FWHM seeing of ~1.4". However, observations made from Anderson Ridge at a location away from any nearby trees indicate that more open sites in that area may enjoy seeing comparable to that measured at Chews Ridge. Simultaneous free-air, ground-level observations at the U.S. Naval Observatory's Flagstaff Station and at Anderson Mesa, both near Flagstaff, AZ, yield similar seeing values to each other (<=0.1" difference).