Experiments to determine concentrations of toxaphene lethal to fishes in a variety of lake types and to discern selective fish-killing properties of the material were conducted. Concentrations ranging from 1 to 85 parts per billion were placed in fourteen bodies of water varying in size 200 from 0.5 to 2,100 acres. Vegetative cover varied from 0 to 95 percent, methyl orange alkalinity from 2 p.p.m. to 74 p.p.m., and hydrogen-ion concentrations from 5.1 to 9.5. Bottom types included gravel, sand, silt and mud. Differences in concentrations required to cause total kills of fish populations in treated lakes appeared to be related to bicarbonate alkalinities, bottom types, amounts of plankton, vegetation and the sizes of fish present.