During the last decade, losses of semi-domestic reindeer Rangifer tarandus have increased in central Norway. Natural mortality in a semi-domestic reindeer herd was studied by use of mortality sensing transmitters. From 15 April 1995 to 15 April 1996,135 of 612 animals equipped with radio collars were found dead. Adult females (>2 years old) suffered a mortality of 18.3%, of which 40.5% was due to predation. Yearlings suffered a yearly mortality of 20.2% of which 66.7% was due to predation. Calf mortality from 6 August 1995 to 15 April 1996 was 31.0%, of which 75.3% was due to predation. Predation by Eurasian lynx Lynx lynx was the predominant cause of death, with 39.3% of the total mortality. Accidents were the second most important cause of death, with 16.0% of the total mortality. Peaks in mortality were registered in September, November and January.