A severe decline in the coastal breeding population of California least terns (Sterna antillarum browni) in California and Baja California (U.S. Fish Wildl. Serv. 1980) prompted both State and Federal governments to designate it an endangered species in 1970 (Massey 1974). Significant losses of nesting and feeding habitat have contributed greatly to the decline of this subspecies (Massey 1974; Atwood and Minsky 1983). 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 (Massey 1974), feeding predominantly on jack-smelt (Atherinops californiensis), topsmelt (A. affinis), and northern anchovy (Engraulis mordax) (Atwood and Minsky 1983). Topsmelt had the highest levels of DDE (p,p'-DDE) (up to 3 #g/g wet wt) of fish collected from San Diego Bay by Ohlendorf et al. (1985). Eggs of Caspian terns ~. caspia) from that study contained up to 56/~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 (Blus 1982). Mercury (Hg) and selenium (Se) have caused decreased hatchability, altered nesting behavior, and embryotoxicosis in birds in field and laboratory studies (Cormors et al. 1975; Ohlendorf et al. 1986; Heinz et al. 1987). Our objective was to evaluate the role of contaminants in the decline of California least terns.