Deer Lodge Valley lies to the northwest of Butte, Montana. It is an early Tertiary structural trough, partially filled with late Tertiary alluvium. A mammalian vertebrate fauna collected from middle Pliocene sediments contains ecological analogues of the modern Deer Lodge ecosystem.1 Sediment distribution suggests similarity of the middle Pliocene pattern of precipitation to the modern pattern. Middle Pliocene habitat zones, species distribution, and population density are considered analogous to those of the modern ecosystem. Recent approximation of the pre-Pleistocene ecological situation testifies to the presence and effectiveness of regional controls that maintained, or preserved the potential to maintain, strong selection pressures during the described geological interval. The replacement of numerous middle Pliocene species by immigrant forms indicates that environmental relationships have played a major role in the reorganization of the valley's mammalian fauna.