Eggs of black-crowned night-herons N. nycticorax were collected for analysis from 7 nesting sites in Colorado and Wyoming in 1979. One egg was taken/nest from as many as 20 nests/site during early incubation. The nests were marked and revisited after hatching, but before fledging, to record the number of live young. DDE was detected in all collected eggs (147) at a mean concentration of 3.1 ppm, fresh basis (residue means were geometric). Mean DDE at the 7 sites varied from 1.8-7.6 ppm. Polychlorinated biphenyls (PCB) were detected in 81 eggs (mean 1.0 ppm); the highest mean at any site was 2.2 ppm. Twelve other organochlorines were each detected in 1-30 eggs, usually at a concentration of lt 1.0 ppm. Mean total organochlorines in the eggs from the 7 sites ranged from 2.0-10.1 ppm and the median number of compounds/egg ranged from 1-5. Eggshell thickness was negatively correlated (P lt 0.001, r = -0.585) with DDE levels in the 147 eggs. Average shell thickness (0.258 +- 0.030 mm) was 8.8% lower than the average thickness (0.283 +- 0.016 mm) of 40 pre-DDT eggs from this region. The nesting sites with the highest DDE and total organochlorine residues in the eggs had the thinnest shells, produced the fewest young and had more nonviable eggs and dead young. At 4 of 7 sites, the average number of live young/nest was lt 2.0, the minimum long-term mean required for population maintenance. The source of the contaminants found in the heron eggs in this study was not determined.