Tests were conducted to determine the effects of dieldrin in continuous-flow water and in food on survival, growth, and bioconcentration in mallard ducklings (Anas platyrhynchos). Dieldrin had no effect on survival or growth at concentrations at or below water saturation (0.2 mg/L), but tissue (lipid, skin, liver, muscle, brain, blood) dieldrin concentrations increased rapidly (up to a mean of 217 microgram/g) and bioconcentration factors (BCFs) showed up to 1,995 times (mean steady-state lipid BCF of 1,345) more dieldrin in tissues than in the water. Significant effects on survival, growth, and behavior were observed in ducks fed dieldrin-spiked food, and tissues rapidly concentrated dieldrin up to five times more than those in the water-only test (mean of 1,000 microgram/g), although the BCFs were much lower (water BCFs were up to 18 times higher). The dietary 96-h LC50 value (50% death at 96 h) was 165 microgram/g dieldrin; the 24-d dietary LC50 was 29.5 microgram/g. The lowest 24-d LOAEL (lowest observed adverse effect level) and NOAEL (no observed adverse effect level) values were 16.4 and 0.3 microgram/g, respectively, based on growth (total weight). A screening level wildlife criteria (SLWC) value of 0.0001 mg/L was derived by using data from the study. Steady-state tissue dieldrin concentrations at which growth impairment occurred were near 140 (lipid), 80 (skin), 20 (liver), 10 (brain), and 5 (muscle) microgram/g dieldrin.