Organochlorine contamination was studied in 8 black-crowned night heron (N. nycticorax) populations nesting in Washington, Oregon and Nevada [USA] in 1978-1980. DDE was detected in 220 eggs sampled; eggshell thickness was negatively correlated with residues of DDE and polychlorinated biphenyls (PCB). Other contaminants were detected in .ltoreq. 35% of the eggs. Except for the 2 Columbia River colonies in which local DDE contamination was a probable compounding factor, a strong north-south clinal pattern of DDE residues among colonies existed. Southern colonies were most contaminated, and productivity was below population maintenance in 1 colony (Ruby Lake). At DDE levels in eggs > 8 ppm, clutch size and productivity decreased, and the incidence of cracked eggs increased. No evidence of breeding-ground DDE-DDT contamination was found except along the Columbia River.