Dicofol is an organochlorine agricultural pesticide used to control mites. The principal commercial dicofol product is known as Kelthane TM. More than 70% of dicofol product (about 3 million Ib or 1.4 million kg) sold annually in the U.S. is applied in California, Arizona, Texas, and Florida. Florida citrus and California cotton receive more than half the total (Clark 1990). In laboratory studies, dicofol, which is structurally similar to DDT, had adverse impacts on reproduction in fish (deformed larvae, delayed hatching), birds (reduced eggshell thickness, reduced hatchability), and mammals (reduced insemination rate, reduced pregnancy rate, failure to produce young) (see Clark 1990 for review). Overall, the reproduction of birds seems less sensitive to dicofol than to DDE. However, in birds, dietary concentrations of dicofol between 1 and 10 µg/g (wet weight) fed to captive adult females caused such problems as eggshell thinning, reduced hatching success, or reduced fertility in American kestrels (Falco sparverius) (Fry et al. 1988, Clark et al. 1990) and eastern screech-owls (Otus asio) (Wiemeyer et al. 1989). In spite of these laboratory findings, there have been no intensive field investigations of possible reproductive effects of dicofol on wild birds. Such studies must wait until field residue data are sufficient to identify populations with high exposure. If dicofol accumulates in birds in the field and heavily exposed populations can be identified, then their reproduction can be studied. Residues have not been reported from reptiles. Analytical screening of wildlife tissue samples for organochlorine chemicals only rarely includes dicofol, and this may explain why the relative hazard of dicofol to wildlife populations is poorly known.