Coloration of pelage is geographically variable. There is a gradual change in the color of the pelage from north to south. Animals obtained in the northern part of the geographic range of _Z. trinotatus_ are generally darker dorsally (more tawny) with the ventral pelage usually pure white. Those individuals from the southern part of the geographic range of _Z. trinotatus_ have the dorsal pelage lighter (more reddish and yellow-brown) and ventrally the pelage is usually heavily suffused with reddish-brown. The crania also vary geographically; they are largest in the northernmost part of the range of the species and smallest in the southernmost part. NATURAL HISTORY _Habitat._--On the Olympic Peninsula, Washington, in 1931 Svihla and Svihla (1933:132) found this species equally abundant in alpine meadows near timberline, in open grassy areas, and in tall meadow grass and low blueberry bushes. All of the mice were in wet marshy places. Bailey (1936:232) reported that in Oregon, these mice live in meadows, marshes, under ferns and weeds in the woods, or near mountain brooks and streams. Taylor (1922:221) found _Zapus_ in moderately moist meadows in the Hudsonian Life-zone at Mt. Rainier, Washington, and Dice (1932:49) found them in deciduous forest and in open, grassy, or sphagnum bogs. Dice records it as common also among the alders and willows in high, open, grassy parks. Merriam (1897b:223) found _Z. trinotatus_ abundantly in moist places grown-over with grass or weeds. Grass cuttings two to three inches long were left in small heaps at feeding sites and indicate the presence of these mice. _Behavior._--Svihla and Svihla (1933:131) write that the long tail of _Z. trinotatus_ is used as a balancing organ when the mouse is in motion. A tailless mouse, attempting to escape, turned somersaults in the air and invariably landed on its back; the loss of its tail seemed to leave the mouse without compensation for the vigorous push of the hind legs. Dalquest (1948:371) noted that the jumping mouse sometimes walks on all fours, but ordinarily moves by means of short hops on the hind feet alone. When startled, jumping mice travel in bounds of six feet or more at a jump. _Zapus trinotatus_, according to Bailey (1936:232) and Elliot (1899:261), is mainly nocturnal but occasionally is active in daylight. Svihla and Svihla (_op. cit._:132) heard captive animals make squeaking noises when fighting. On several occasions captive animals made a drumming noise by rapidly beating the tail against a resonant body such as the bottom of a tin can.