Rural land in the U.S. Rocky Mountains is being subdivided and developed for residential and commercial use at an unprecedented rate. In the East River Valley, Colorado, land ownership parcel size began to decline sometime during the last two decades after increasing for over a century-since the first European settlement. Roughly 20% of the private land in the valley has been divided into parcels smaller than about 45 acres (18 ha), parcels that cannot economically support agriculture. Development in the valley has caused significant land-cover fragmentation, especially in aspen, meadow, mixed conifer, and sagebrush cover types. Increased rates of house construction, from 3% per year during 1964-1990 to 8% per year from 1990 to 1994, and the conspicuousness of new roads and houses on the valley slopes, have enhanced local perception of rapid landscape change. Residents' concern over land use especially focuses on the creation of "ranchettes"-large-lot residential areas carved out of former cattle ranches-and on the community changes associated with population growth, absentee home ownership, and class distinctions between recent immigrants and long-time residents. This paper documents these changes in the ecological and social landscape of this rapidly developing mountain valley using landscape metrics and interviews of key residents.