The flow cytometry method (FCM) was employed to determine cellular DNA content of black-crowned night-heron (Nycticorax nycticorax) embryos and 10-day-old chicks collected at sites differing in types of chemical contamination. The coefficient of variation of DNA content (CV) in blood collected from embryos suggested cytogenetic damage at a site in Louisiana known to be contaminated with petroleum. Blood CV from chicks suggested genetic damage at a site in Texas also known to be contaminated with petroleum. Spleen CVs in chicks were significantly lower than respective means from the reference site. The CVs of chick blood and liver and spleen negatively correlated, suggesting recovery of spleen and liver cells after exposure to a clastogenic compound. Thus, the lower CVs may also have been indicative of genetic damage. Based on the findings of this study, FCM is a potential indicator of certain environmental contaminants in black-crowned night-herons.