Several species of herons, which are top-level consumers in aquatic food chains, have experienced population declines in certain areas o f their normal range (7,13) -- areas in which elevated levels of various environmental pollutants are known to occur. (6) To determine the effects of environmental contaminants on the Ardeidae, a colony of black-crowned night herons (Nycticorax nycticorax) was established in 1972 at the Patuxent Wildlife Research Center. The night heron was selected as the model species because of its widespread occurrence and its ability to survive and reproduce in captivity. Birds for the colony were obtained from either the New York Zoological Park and Dallas Zoo or were wild-caught along the Maryland and Virginia coasts in 1972, 1973, and 1975. This report describes a die-off in the colony following a change in the origina of their food source. The data suggest that the mortality was diet-related, most likely caused by vitamin E deficiency. Excessive dietary thiaminase may have resulted in concurrent thiamine deficiency, but evidence for this is equivocal.