From May 1975 to June 1977 aspects of the ecology of the fisher were studied in the field by use of radio-telemetry, tracking, and track observations. Information on reproduction, food habits, age and sex distributions, and physical condition was gathered from analysis of 195 trapper-caught fisher carcasses collected from four areas durinq the winters of 1972-73 to 1971-78. During the winters of 1975-76 and 1976-77 the upper levels of the snow cover affected fisher locomotion and extensiveness of activity. Chanqes in the use of gaits and cover types were qualitatively correlated with physical variations in the snow cover. An almost complete depression of fisher activity occurred in the intensive tracking unit during periods in January and February, 1975-76 and January, 1976-77 when the area was covered by extremely soft snow. Effects of temperature, prey availability, and social behavior could not be separated absolutely from the effects of snow cover, but application of snow factors into an established ecological energetics model developed to estimate energy requirements of free-ranging fishers indicated that soft snow could greatly increase energy expenditure of active fishers. The response of fishers to snow cover was believed to be unique for boreal Martes. Information from age class structure and radio-telemetry indicated that the juveniles moved more extensively than adults during winter...