A biological survey of Great Slave Lake was begun in 1944 in order to estimate its capacity for fish production before commercial fishing began. This great inland lake of 10,500 square miles was found to have a fairly heavy population of desirable fish with lake trout, lake whitefish, and ciscoes predominating. Physical, chemical, and biological evidence suggested a capacity for sustained production not unlike that of the upper Great Lakes and Lake Nipigon. A first estimate of 3 to 5 million pounds per year was made in December 1944. Commercial fishing began in 1945 and has now increased to a point where production is 3.7 million pounds per year. General biological studies extended over 4 years, and a continuing program has been established to follow the effects of fishing on the fish population. This lake affords a unique opportunity for the scientific investigation of a large fishery and for the conservation of an important natural resource.