Condors are sensitive to marine contaminants in the environment. We conducted research in collaboration with Robert Riseborough (The Bodega Bay Institute), U.S. Fish and Wildlife Service, Santa Barbara Zoo, and the Los Angeles Zoo, and found that mean thickness of eggshell fragments in central California nests was 34% lower than the mean for interior southern California. We also found lower hatching success in central California. We concluded that eggshell thinning, characterized by a reduction or absence of the outer crystalline layer, was associated with marine contaminants, because condors in central California frequently scavenge marine mammal carcasses, whereas condors in interior southern California do not. We found that DDE, a metabolite of the infamous organochloride DDT, was the source of condor eggshell thinning based on the concentrations measured in our sample of failed eggs and the proximity of the Montrose Chemical Corporation DDT contamination site to seasonal sites used by marine mammal populations visiting the central California coast. We can help mitigate the threat by replacing thin-shelled eggs discovered in nests with normal eggs laid by condors in captivity.