A study has been made of the toxic substances which may be found in kraft pulp‐mill waste‐waters. It has been determined that the sulphides, mercaptans, resin acid soaps, and sodium hydroxide constitute the greatest hazard. The minimum lethal concentration of these and other materials to fresh‐water minnows, Daphnia, and aquatic insect larvae has been established. Methods have been devised for evaluating these materials in kraft‐waste waters, and data from the examination of the wastes of a typical northern kraft mill are presented.