The insecticide fenitrothion is sprayed twice, 5 to 10 d apart, to control spruce budworm. To assess the effects of pulsed exposure of fenitrothion to different species of amphibians, we exposed 1- and 8-d-old tadpoles of wood frogs, leopard frogs, green frogs, bullfrogs, and American toads, as well as larvae of the spotted salamander, to 2 to 9 ppm of the insecticide for 24 h. Our results indicate that within each ranid species, day-1 and day-8 tadpoles are equally sensitive, and tadpoles exposed twice, on day 1 and day 8, are no more sensitive to the second exposure than to the first. Bullfrog and green frog tadpoles are more sensitive to exposures than are wood frog and leopard frog tadpoles, exhibiting paralysis at lower concentrations, and they are affected by exposure to the carrier compounds as well as the insecticide. The greater sensitivity of the green frog and bullfrog tadpoles correlates with their smaller size and earlier developmental stage at hatching. American toad tadpoles are more tolerant of pulsed exposure to fenitrothion than are any of the ranids, whereas the spotted salamander larvae are as sensitive as are the bullfrog and green frog tadpoles.