A severe decline in the coastal breeding population of California least terns (Sterna antillarum browni) in California and Baja California prompted both State and Federal governments to designate it an endangered species in 1970. Significant losses of nesting and feeding habitat have contributed greatly to the decline of this subspecies. However, environmental contaminants, such as organochlorine compounds and metals, may also have contributed to the decline. California least terns are primarily piscivorous during the nesting period, feeding predominantly on jack-smelt, topsmelt, and northern anchovy. Topsmelt had the highest levels of DDE (p,p`-DDE) (up to 3 {mu}g/g wet wt) of fish collected from San Diego Bay. Eggs of Caspian terns (S.caspia) from that study contained up to 56 {mu}g/g DDE, and DDE was associated with a reduction in eggshell thickness as determined by the thickness index. In addition to shell deficiencies, organochlorines can also cause reduced egg production, aberrant incubation behavior, delayed ovulation, embryotoxicosis, and mortality of chicks and adults. Mercury (Hg) and selenium (Se) have caused decreased hatchability, altered nesting behavior, and embryotoxicosis in birds in field and laboratory studies. Our objective was to evaluate the role of contaminants in the decline of California least terns. 22 refs., 1 fig.,more » 2 tabs.« less