_Description._--The otter has the long body of many mustelids, but is specialized for aquatic life. Males are slightly larger than females. Large adults measure more than 3-1/2 feet in length, of which the tail comprises 18 inches. The largest adults may weigh as much as 25 pounds. The body is elongate but rather plump. The forelimbs are small and laterally placed. The hind legs are the heavier, and are situated posteriorly. The feet are large and webbed for swimming. The tail is long, thick at the base and gradually tapering to a narrow tip. The head is small and rounded. The ears are tiny and the eyes are of moderate size. The upper lips are large, somewhat spaniel-like, and support a mustache of stiff vibrissae. The fur is short, dense and soft. The tail is sleek with short, posteriorly-directed fur, and is not bushy. The fur is rich, dark chocolate brown in color, slightly paler on the animal's underparts than on its upper parts. Otters range over Eurasia, North America and South America. Those occurring in North America north of Mexico are thought to belong to a single species, _canadensis_. Several species from tropical America may eventually be shown to also be races of _canadensis_. The aquatic habits of the otter allow it to exist in several life-zones, in both marine and fresh-water habitats. The principal environment is the streams, rivers and lakes of the Transition Life-zone, but along the larger rivers, otters range up into the Canadian Life-zone and along the Snake and Columbia rivers they enter the Upper Sonoran Life-zone. They are active throughout the year. As we have observed them, they are principally nocturnal. Otters are extremely powerful swimmers. Tracks along the North Fork of the Tolt River, King County, showed where otters had entered water that flowed over a stony bottom at the velocity of rapids. Their occurrence in the Tolt, Skykomish and Snoqualmie rivers where these are swift mountain streams indicates exceptional swimming ability. After emerging from the water, an otter often follows along the shore, sometimes for miles.