The net plankton of Great Slave Lake was studied extensively in the years 1944 to 1947 and sampling continued at a central station through 1954. Some 490 samples were taken using large nets and traps with No. 20 silk. Taxonomic, gravimetric and numerical analyses are reported and comparisons made between the larger, open lake and the deep, cold, east arm.Among 160 species of algae, two diatoms,Melosira islandicaandAsterionella formosawere dominant. Green and blue-green algae were scanty butDinobryonwas numerous, especially in the cold, east arm. Among 26 rotifersKeratella cochlearisandKellicottia longispinawere dominant and four other species common. Cladocera were scarce but the copepods made up about 85% of the plankton volume.Diaptomusspp.,Cyclopsspp.,Limnocalanus macrurus,Epischura lacustrisandSenecella calanoideswere the main species.The average dry weight of the standing crop of net plankton in the open water was 21.8 kg./ha. The crop was heavier in years of warm water than in those of colder water. Occasional high turbidity was associated with decreased plankton. In Christie and McLeod Bays of the east arm, where the waters are deeper, colder and lower in mineral content, the average dry weight of plankton was 14.3 and 9.0 kg./ha. These amounts are typical of the plankton in large, very oligotrophic lakes.An early season maximum of diatoms and copepods was followed by a rapid decline in late July. The smaller, late-summer maximum included large numbers of rotifers. More than half of the plankton was in the upper 25 metres with decreasing amounts down to 100 metres. A thin plankton, mainly of copepods, extends down to 600 metres.The dominant species of plankters in the open water of Great Slave Lake appear to be almost identical with those of Lake Winnipeg, Lake Nipigon and the Great Lakes. All have a diatom-copepod type of plankton with a constant group of rotifers andDinobryonin moderate numbers. Green and blue-green algae are scanty except in the shallower and warmer lakes, Winnipeg and Erie.