Four adult (2M:2F) snow leopards (Uncia uncia) were radio-monitored (VHF; one also via satellite) year-round during 1994-1997 in the Altai Mountains of southwestern Mongolia where prey densities (i.e., ibex, Capra siberica) were relatively low (-0.9/km(2)). Marked animals were more active at night (51%) than during the day (35%). Within the study area, marked leopards showed strong affinity for steep and rugged terrain, high use of areas rich in ungulate prey, and affinity for habitat edges. The satellite-monitored leopard moved more than 12 kin on 14% of consecutive days monitored. Home ranges determined by standard telemetry techniques overlapped substantially and were at least 13-141 km(2) in size. However, the satellite-monitored individual apparently ranged over an area of at least 1590 km(2), and perhaps over as much as 4500 km(2). Since telemetry attempts from the ground were frequently unsuccessful (x = 72%), we suspect all marked animals likely had large home ranges. Relatively low prey abundance in the area also suggested that home ranges of >500 km(2) were not unreasonable to expect, though these are >10-fold larger than measured in any other part of snow leopard range. Home ranges of snow leopards may be larger than we suspect in many areas, and thus estimation of snow leopard conservation status must rigorously consider logistical constraints inherent in telemetry studies, and the relative abundance of prey. (c) 2005 Elsevier Ltd. All rights reserved.