PESTAB. Physiological selectivity of insecticides comes into play when equivalent contact between different organisms and the same toxicant result in different acute toxicities. It is related to differences in the morphology and biochemistry of the organisms compared and, therefore, bears directly on the mode of action and design of insecticides. Specific topics discussed in this review of selectivity include: biodynamics and selectivity with reference to observed toxicity and intrinsic activity, the dynamics of toxicant behavior in an insect or other animal, detoxication and syngerism of various aryl N-methyl carbamates, penetration through the integument and gut, circulation and distribution, selectivity at the site of action, metabolism and selectivity with emphasis on the principles of metabolism, and the influence of molecular structure. Specific insecticides discussed include: malathion, acethion, dimethoate, parathion, tetrachlorvinphos, carbaryl, DDT, chlordane, sesamex, dieldrin, lindane, pyrethroids, rotenone, nereistoxin and juvenile hormone and juvenile hormone mimics.