R. Zweifel April 8, 1951 271 Rana boylii 7.1 mi 556 Clayton, Contra Costa Co., Calif. One-half mile of stream was covered, working most of one side and part of the other. In this 880 yards of stream, only 7 Rana boylii were seen, one of which was captured. This adds up to about one frog for every 377 feet of stream, which seems to be a rather low population density. Probably this condition may be attributed to the scarcity of trees and bushes beside the stream, which reduces the available insect food. Rana boylii seems to be able to better invest the current than does R. aurora, when it is possible to watch the two swimming under similar conditions.