Pesticides are commonly used for health and economic benefits worldwide, but increased use has led to increased contamination of aquatic habitats. To understand potential impacts on nontarget organisms in these habitats, toxicologists generally use short-term (4-d) toxicity tests on model organisms. For most pesticides, few amphibian tests have been conducted, but there is growing concern about the potential impact of pesticides to amphibian populations. For the insecticide endosulfan, previous studies have found that low concentrations can be very highly toxic to amphibians and have suggested that this mortality may exhibit important lag effects. To estimate the lethal concentration of endosulfan that would cause 50% mortality after 4 d (LC504-d) across a diversity of amphibians and the presence of lag effects, LC504-d experiments were conducted on nine species of tadpoles from three families (Bufonidae: Bufo americanus, B. boreas; Hylidae: Pseudacris crucifer, P. regilla, Hyla versicolor, and Ranidae: Rana pipiens, R. clamitans, R. cascadae, R. catesbeiana) and then held the animals for an additional 4 d in clean water. The LC504-d values for endosulfan ranged from 1.3 to 120 ppb, which classifies endosulfan as highly toxic to very highly toxic. Moreover, holding the animals for an additional 4 d in clean water revealed significant additional mortality in three of the nine species. Leopard frogs, for example, experienced no significant death during the initial 4-d exposure to 60 ppb but 97% death after an additional 4 d in clean water. A phylogenetic pattern also appears to exist among families, with Bufonidae being least susceptible, Hylidae being moderately susceptible, and Ranidae being most susceptible. Results from the present study provide valuable data to assess the impact of endosulfan on a globally declining group of vertebrates.