This species of meadow mouse ranges from Canada south to Colorado in the Rocky Mountains and in the Cascades south into Oregon. The water rats are strictly alpine animals, occurring about streams, marshes and damp meadows. In the Cascade Mountains they are most common in the glacial cirques where tiny streams flow through grassy meadows to plunge over the lip of the cirque on a rocky course of cascades to the valley below. Here their broad trails occur along the stream banks, commonly entering the water where it is swift. These trails are well-worn roads, usually about four inches wide but often wider. The burrows of the water rat are about three inches in diameter and are constructed with no effort at concealment; large mounds of earth mark their entrances. Freshly dug burrows are so abundant that it seems likely more are dug than are actually inhabited. Burrows are often dug beneath rocks. The water rat is mainly nocturnal but not uncommonly is seen in the daytime. Twice I cornered a water rat away from its burrow and each time it escaped by swimming. The rats swam with great speed but with much splashing. One dived under the surface of a small pool and disappeared. Stones along the bank were pulled out until the rat was captured in a small chamber at the end of a burrow. The burrow entered the bank at the base of a large stone six inches beneath the surface. This burrow resembled a miniature muskrat burrow and apparently had been dug when the rat was under water. Like _Phenacomys_, the water rat constructs sub-snow nests on the surface of the ground. These are recognizable by their large size and by piles of ovoid droppings a quarter of an inch in length. These nests are loosely built and fall apart soon after the snow melts. The water rat is sometimes a pest to the mammal collector for they spring mouse traps set for other mammals without becoming caught. At times the greater part of an entire trap line was thus rendered ineffective by these mammals. Fully adult animals are uncommon in collections for a rat trap or steel trap is needed to take them and these items, when packed on back up mountains to water-rat habitat, are usually set for still larger animals. Racey (Racey and Cowan, 1935: H27) recounting his difficulty in securing specimens when no suitable traps were at hand, writes: "Killed one with my hands, shot another, and a third was stunned by a mouse trap." Taylor and Shaw (1927: 76) list food eaten by the water rat on Mount Rainier as the avalanche lily, dogtooth violet, _Ligusticum purpureum_, _Valeriana sitchensis_, _Polygonum bistortoides_, _Petasites frigida_, _Phyllodoce empetriformis_, _Potentilla flabellifolia_, _Aster_ sp., grass, wild clover, conifer seeds, two kinds of blueberry (_Vacinnium_) and _Xerophyllum tenax_. Racey and Cowan (1935) list foods eaten in the Cascades of southern British Columbia as _Lupinus polyphyllus_, _Senecio balsamitae_, _Pedicularis bractiosa_ and _Arnica alpinus_.