During spring 1977 and 1978, 136 moose (Alces alces gigas) calves were radio-collared in the Nelchina and Susitna river basins of south central Alaska in an effort to determine causes of mortality. Thirteen calves (9.5%) died as a result of collaring activities. Of the 123 remaining calves exhibiting normal cow-calf bonds, contact with 3 calves was lost and 66 (55%) died of natural causes. Predation by brown bears (Ursus arctos) was the most important cause of mortality, accounting for 79% of the deaths. Timing of the deaths of radio-collared calves was similar to that of uncollared calves of the radio-collared adults, which indicated that collaring did not predispose the calves to predation. Ninety-four percent of the natural mortality occurred before 19 July each year. Little scavenging of either abandoned or predator-killed calves was observed. Radio-collared brown bears were observed on 78 kills during 1978, averaging 1 ungulate kill/6.1 observation days. Identification of brown bear as a significant predator of moose complicates attempts to understand and manage ungulate-carnivore relationships.