Irruptive populations of Peromyscus maniculatus occasionally have been reported, but generally no data on population structure are available. A very dense population was encountered in June, 1954, in the White Mountains of Mono County, California. In a period of eight days of snap trapping in Cottonwood Creek basin, 9500 ft. elevation, 528 P. maniculatus sonoriensis were taken in 1204 trap nights. Animals were classified as to sex and age class (adult or juvenile). During the first two or three nights, the catch rate closely approached or attained one mouse per trap night; thereafter the rate declined. Other small mammals trapped in this period included 10 Microtus longicaudus , 15 Microtus monianus , …