Mammalian predators have been liberated on many islands to control pests but the desired results have rarely been achieved. The liberation of mongooses on islands in the Caribbean and Pacific is an excellent example of a liberation which had serious repercussions. These were liberated to control rats which caused serious damage in the sugar cane fields and although they reduced the numbers of rats, they also exterminated many species of small mammals and birds. Stoats, ferrets and weasels were liberated in New Zealand in the early 1880's to control rabbits. They appear to have had little effect on rabbit populations and stoats quickly spread into forest areas. It is not now possible to determine the effect these predators had on the native bird populations as the changes in fauna and habitat were complex. Unfortunately no studies of mustelids were made until 1948 when Wodzicki (1950) made a brief study as part of his survey of introduced mammals in New Zealand. The two periods of greatest change for mustelids have been during their spread throughout the country in the 1880's and in the early 1950's when rabbits were successfully controlled. There is little information on changes in density or feeding habits of mustelids during these times. Detailed investigation of the ecology of mustelids in New Zealand was begun by Dr W.H. Marshall, Fulbright Research Scholar from the University of Minnesota, with Animal Ecology Division, D.S.l.R. from September 1960 until June 1961. He examined their ecology in the light of his experience of mustelids in North America where conditions differ markedly from those in New Zealand. I joined Animal Ecology Division in November 1960 to assist Dr. Marshall throughout the remainder of his study, and continued the work after his return to the United States. The ecology of stoats, ferrets and weasels has been investigated in terms of their adaptation to food supplies which differ markedly from those in their native range in the Northern Hemisphere.