Tests were conducted to determine (1) the acute toxicity of dieldrin in flowing sea water to American oysters (Crassostrea virginica), pink shrimp (Penaeus duorarum), grass shrimp (Palaemonetes pugio) and sheepshead minnows (Cyprinodon variegatus) and (2) the rate of dieldrin uptake and depuration by spot (Leiostomus xanthurus). Acute (96-hour) EC50's were: oysters, 12.5 ug/1; pink shrimp, 0.9 ug/l; grass shrimp, 11.4 ug/1; and sheepshead minnows 23.6 ug/I. Spot exposed to 0.0135,0.075,0.135,0.75 or 1.35 ug/l for 35 days accumulated the chemical with maximum concentrations attained in II to 18 days. Maximum whole-body residue (wet-weight) was 6,000X the concentration in test water. Spot contained no detectable dieldrin residues at the end of a 13-day depuration period in dieldrin-free water. Tissue alterations, such as subepithelial edema in gill lamellae and severe lysis and sloughing of the small intestine epithelium, occurred in spot exposed to 1.35 ug/1 for four days.