Chlorpyrifos is a broad-spectrum organophosphorothioate insecticide with a principal mechanism of toxicity by inactivation of acetylcholinesterase at nerve junctions. Unlike certain organochlorine pesticides, chlorpyrifos is relatively nonpersistent (Racke 1993), and its principal degradation products are less toxic than the parent chemical. Species sensitivity varies considerably across kingdom and phyla. In general, aquatic and terrestrial microorganisms and plants are tolerant to chlorpyrifos exposure. Aquatic invertebrates, particularly crustaceans and insect larvae, are sensitive to exposure: LC50s are generally less than 1 microgram/L, and no-observed-effect concentrations may be below 0.1 microgram/L in laboratory studies. Fish appear to be less sensitive, with LC50s generally between 1 and 100 micrograms/L and no-observed effect concentrations of approximately 0.5 microgram/L. In general, saltwater and freshwater organisms exhibit similar sensitivity to chlorpyrifos, considering the extreme phylogenetic and species differences in toxicity. Chlorpyrifos effects in aquatic ecosystems are complex because of the diversity of species assemblages and trophic interactions. In general, functional endpoints (e.g., community metabolism) are less sensitive than structural parameters of ecosystems (e.g., loss of sensitive species). Ecosystem recovery is dependent on the interaction of a variety of factors including treatment timing and application dose, rate of dissipation, species assemblages, trophic structure, and the reproductive capacity and growth rate of susceptible and tolerant populations. Terrestrial species are relatively tolerant of chlorpyrifos exposure, although contact toxicity to sensitive terrestrial invertebrates may occur at concentrations of 0.1 microgram/insect. Amphibians, birds, and mammals show similar sensitivity to orally administered chlorpyrifos, with LD50s ranging from 8 to > 400 mg/kg body weight. Long-term chronic feeding studies in birds and mammals have shown no observed effect concentrations to be greater than 1 mg/kg food. In general, field studies have shown limited or no acute toxicity to amphibians, reptiles, birds, or mammals.