Laboratory experiments were conducted to measure uptake of fission products by fish from water and from oral doses. Availability of fission products to the fish was curtailed considerably by the formation and settling of radioactive particles in the water and by sorption to abiotic surfaces. Spot, Leiostomus xanthurus, and Atlantic croaker, Micropogon undulatus, accumulated relatively low levels of radioactive material from seawater. Following a single oral dose, croaker assimilated a maximum of 14% in 4 hours, but 94% of the original dose was eliminated after 4 days. Bluefish, Pomatomus saltatrix, assimilated only 3% of the cumulative dose of mixed fission products administered in shrimp meat on alternate days for 55 days. Of the amount assimilated from oral doses by croaker and bluefish, the largest concentrations were found in the internal organs.