Dicofol is an organochlorine acaricide structurally similar to DDE and there is concern that it may effect avian reproduction. Effects of dietary dicofol on mallard (Anas platyrhynchos) egg production and eggshell quality were evaluated; the mallard is moderately sensitive to DDE shell-thinning toxicity. Hens in egg production were fed either 0, 3, 10, 30, or 100 microgram/g dicofol for 42 days (the formulation of dicofol contained <0.1% DDT-related impurities). A positive control study was also conducted in which mallard hens in egg production were fed 0, 3, 10, and 100 microgram/g DDE for 42 days. Egg weight, shell thickness, shell strength, and dried shell weight were measured for all eggs produced. Egg production was not affected by dicofol diets but the percentages of cracked and soft-shelled eggs from birds on the 100 microgram/g dicofol diet were significantly (p <0.05) greater. Shell strength, thickness, and weight were negatively related to dicofol dietary concentrations.