Wildlife casualties from aldrin-dieldrin poisoning are associated with the planting of aldrin-treated rice seed along the Texas Gulf Coast. The fulvous tree duck (Dendrocygna bicolor), which depends on the rice field habitats and is highly susceptible to aldrin-dieldrin poisoning, is suffering a serious population decline in that area. Dead waterfowl, shorebirds, and passerines were collected on study areas in Wharton, Brazoria, and Chambers counties, Texas, from 1967 through 1971. Residues of aldrin or dieldrin were found in all samples of bird casualties and in all eggs, scavengers, predators, fish, frogs, invertebrates, and soils. Fulvous tree ducks appeared to be less resistant to aldrin than other ducks. Dieldrin residues in brains of dead fulvous tree ducks were low, but whole-body residues were as high as 16 ppm. Brains of other dead ducks and geese averaged 10 ppm dieldrin. Some dead birds were exposed by eating treated rice seed, but many dead birds with high dieldrin residues were species that feed largely on invertebrates. Although soil residues were low, snails and crayfish contained enough aldrin and dieldrin (average 9.5 ppm) to account for deaths in birds that fed heavily on these invertebrates over a period of time. When fulvous tree ducks were penned for 3 days in fields aerially planted with treated seed, 3 of 10 birds died with brain residues of 2.5, 2.9, and 6.8 ppm dieldrin, and others were intoxicated. None of eight died, and some gained weight, when penned in fields planted with untreated seed. This study adds further evidence for the suspected lethal effects of aldrin-treated rice seed on wild birds and other wildlife in rice field habitats.