Reproductive success of a colony of common egrets (Casmerodius albus) in California declined between 1967 and 1970. Successful nesting attempts decreased from 52 to 28%, and nests losing eggs increased from 30 to 54%. Reproductive success of great blue herons (Ardea herodias) in this colony showed no comparable trends over this period. Mean thicknesses of all egret and heron eggshells recovered from the floor of the heronry were 15·2 and 10·4% lower, respectively, than pre-1947 values. Eggs of both species, broken during incubation, were 17% below normal thickness. Concentrations of DDE recorded in egg lipids and in the carcasses of adults found dead or moribund were comparable with those found in other species now producing thin-shelled eggs. Polychlorinated biphenyls ranged up to 15 ppm in the brain and 93 ppm in the livers of adult egrets, and concentrations of total mercury in the liver ranged between 2 and 9·5 ppm. Dieldrin concentrations in the brains of four adult egrets found dead or moribund ranged between 5 and 7 ppm, suggesting death by dieldrin poisoning. Ratios of DDE to PCB in the more heavily contaminated egrets indicated that the birds had acquired the major part of their body residues on the wintering ground rather than in the immediate vicinity of the colony.