The preferred habitat ofReithrodonin northern Patagonia is grassy turf, where densities reach about 10 per ha. It occupies burrows in the daytime and feeds on green vegetation at night. The unwary nature of many individuals permits close observation for hours at a time from a distance of only a few meters. They feed for many hours each night, flagrantly exposed to numerous competent predators. Longevity is less than 2 years, and reproductive potential is similar to that of other mice living nearby. Females are as large as males. Each night reithrodons eat their own weight in green grass and produce hundreds of fecal pellets; apparent digestibility is 43 percent. A singleReithrodonrequires the standing crop on about 0.13 m2of turf each night.