Found at about 10,160' elevation on the south flank of Sonora Peak, a few kilometers up the trail north of Sonora Pass. Growing nearby were both Hydrophyllum occidentale and Phacelia hastata ssp. compacta...both were in flower, though the latter was more abundant.

Using the (virtually identical) keys in Richards(1963) and Richards(1966), this goes to either Pseudomasaris zonalis (if you go, at lead 3, with "Mid femur flattened beneath and with a more or less distinct anteroventral ridge, distinctly wider at base than at apex"), or to P. macneilli (if you go with ""Mid femur rounded beneath with no anteroventral ridge, scarcely wider at base than at apex"). In Bohart's description of P. macneilli, appearing on on pg. 292 of Richards(1963), the following data is given:

"Holotype male.—Near Sonora Peak (above Sonora Pass), Tuolumne County, California, 11,000 feet elevation, July 14, 1957 (C. D. MacNeill, C.A.S., San Francisco).
On the same page, Richards' discussion of P. macneilli includes the following:

"In Sonora Pass, on June 29, 1961, I found this species to be not very common, at about 10,000 feet on the Tuolumne side, on or around flowers of Hydrophyllum. This was on the sunny east-facing side of a small valley on the north of the road and no other species was present. On the next day, rather more than a thousand feet lower down, P. zonalis (Cresson) was visiting the flowers of Phacelia frigida Greene, on the edge of the road. In 1960, between August 10 and 18, a party from the University of California (Davis and Berkeley campuses) collected on Sonora Pass. They obtained 7 males and 55 females of P. zonalis, but earlier (June 26, 1960) Mr. R. W. Thorp obtained one female of P. macneilli (now in his collection)."
And Richards adds:

"P. macneilli [...] could, in the female, be confused with P. zonalis, especially since the anteroventral ridge on the femur of the last-named species is not always conspicuous. P. macneilli, however, is larger, yellower (especially beneath the gaster), and has longer, hairs and shorter propodeal spines."
In light of Richards comments quoted above, comparing the description of P. macneilli in Richards(1963) with the original description of P. zonalis, I couldn't come up with a clearly discernible character in my photos to distinguish which of the two species we have here (though the description of the coxae and trochanters being black for zonalis vs. with yellow spots for macneilli seems to favor the former here a bit). Since Richards comments indicated zonalis was the more commonly encountered species, and its August 10 collection date is closer to my observation date than Robbin Thorp's June 26 collection of macneilli, I guess zonalis is the most likely ID here. But this might be P. macneilli...I can't say for sure.