The first commercial pesticide derived from a class of compounds known as halogenated pyrroles was registered for use in the United States in 2001. Chlorfenapyr degrades slowly in soil, sediment, and water and is highly toxic to birds. Information on biochemical or histological endpoints in birds is lacking; therefore, a two-year study was conducted to provide information needed to develop diagnostic criteria for chlorfenapyr toxicosis. In the first year, male mallard ducks were fed concentrations of 0, 2, 5, or 10 ppm technical chlorfenapyr or 5 ppm of a formulated product in their diet during a 10-week chronic exposure study. Survival, body weight, feed consumption (removal), behavior, and molt progression were monitored. Feed and liver were analyzed for chlorfenapyr and two metabolites. Five of 10 ducks in the 10-ppm group died, and neurotoxic effects were observed in the 5- and 10-ppm groups. Feed removal increased for ducks receiving chlorfenapyr and body weights of 5- and 10-ppm ducks were reduced. Loss of body fat, muscle atrophy, and bile retention were suggestive of metabolic disruption or a decreased ability to digest and absorb nutrients. Liver and kidney weights and liver and kidney weight/body weight ratios exhibited a positive response to concentrations of chlorfenapyr in the diet. Emaciation and elevated organ weight/body weight ratios are candidates for a suite of indicators of chronic chlorfenapyr exposure. Liver is the preferred tissue for chemical confirmation of exposure.