Primarily covered in dark-brown fur, it is found near aquatic habitats along the Pacific coast from southern British Columbia to northern California.

With air trapped in its fur for buoyancy, marsh shrews can run for three to five seconds on top of the water.

The marsh shrew is the largest member of the genus Sorex in North America,[2] and mammalogist David Nagorsen described it as "an attractive mammal".

[9] It was first described in the scientific literature in 1884 by Clinton Hart Merriam with its original name, Atophyrax bendirii (a monotypic taxon at the time).

Variations in the sequencing of cytochrome b mitochondrial DNA were assessed, and the results of the 2005 phylogeny for the marsh shrew are shown below in detail.

These specifications have historically been based on fur markings, skull shape and dental details of unclear significance, and their validity is uncertain.

[12] The geographic range of the marsh shrew extends from southwest British Columbia, along the western regions of the U.S. states of Washington and Oregon and through northwestern California to the area north of San Francisco.

[1] During cold, rainy seasons, they may travel as much as a kilometer from wet areas to more sheltered habitats;[14] these generally include mixed deciduous or coniferous forest with downed logs and surface cover.

[16] In 1992, Carlos Galindo-Leal and Gustavo Zuleta trapped 1,000 small mammals at 55 locations in a large area of southwestern British Columbia; only three were Pacific water shrews.

[3] During spring and summer 1983, biologists in western Oregon studied small-mammal distribution (including marsh shrews) near streams and along the riparian fringes of coniferous forests.

All the marsh shrews were trapped at streamside, and were found in all three ages of coniferous forests: old-growth, mature and young growth.

[17] The marsh shrew eats invertebrates, including spiders, earthworms, sowbugs, centipedes, termites and other terrestrial and aquatic arthropods.

[18] A study of the gastric contents of marsh shrews in Oregon indicated that at least 25% of their diet is aquatic,[19] including insect larvae, slugs and snails, mayfly naiads and other, unidentified invertebrates.

[3] The marsh shrew swims, making short dives in search of food;[1] its mobile snout, whiskers and lips are used to find and capture underwater prey.

[14] Air trapped in its fur provides buoyancy, and marsh shrews can run for as long as 3 to 5 seconds along the top of the water.

Marsh shrews typically live about 18 months,[1] and males are not thought to reach sexual maturity during their first summer.

[22] Marsh shrews are easily trapped in sunken cans, possibly due to their inability to see where the edges of surfaces drop.

[21] In captivity they vocalize when they are displaced or scuffle with other animals in their cage,[14] twittering shrilly if disturbed while eating or in a confrontation over food (such as a worm).

[15] Acarine parasites include the Glycyphagidae (Glycyphagus hypudaei and Orycteroxenus soricis); the Laelapidae (Androlaelaps fahrenholzi, Echinonyssus obsoletus, Haemogamasus occidentalis and Haemogamasus reidi); the Listrophoridae (Listrophorus mexicanus); the Myobiidae (Amorphacarus hengererorum, Amorphacarus soricis, Protomyobia atophyracis and Protomyobia brevisetosa), and the Pygmephoridae (Pygmephorus horridus and Pygmephorus whitakeri).

Their available habitat continues to degrade as a result of economic activity in the area; with little chance of the trend reversing, they are rare in that part of Canada.