Palmer 1937 Reithrodontomys megalotis Line. S Antioch, 100 ft., Contra Costa Co., Calif. March 7 Caught 3 in 40 gr.-size mouse traps. All were taken in well made runways in grass, mixed green and dry, 6"- 18" tall. Most of the runways ran along beneath the lower wire - which was about 5 inches off the ground of a barbed wire fence. All traps set along the side of the road outside a field of young grain. It is probable that the runways were, for the most part, made by Microtus, since they were larger than any I have seen which appear to be used by Reithros. only.