Arsenic analyses were made of over 1,000 samples of water, plankton, soil and fish from a 22-acre, a 26-acre, and eight 0.25-acre earthen ponds that were treated with different amounts of sodium arsenite. The concentrations of arsenic in the waters from the 22- and 26-acre ponds treated with 4.0 p.p.m. As203 in the fall of 1954 declined to 0.05 p.p.m. As203 by January, 1955. The concentration of arsenic in the water of the 22-acre pond that was again treated with 4.0 p.p.m. AS203 in April, 1955, declined to 0.8 p.p.m. AS203 one week after application; after this pond was drained and refilled one time during the winter of 1955, no arsenic was found in the water. Four 0.25-acre ponds received arsenic in 1955. Even after these ponds were drained and refilled three times, considerable amounts of arsenic were retained by the bottom soils and were slowly released into the water each time they were refilled. Six weeks after the third refilling, up to 0.3 p.p.m. AS203 was found in the water and up to 714.3 p.p.m. AS203 on a dry weight basis was found in the plankton. The O.25-acre ponds received an application of approximately 4.0 p.p.m. As2O3 July 24, 1956. Seventy-two hours after treatment, the arsenic appeared uniformly mixed throughout the water, averaging 4.2 p.p.m. As203. The concentration of arsenic declined rapidly to 1.7 p.p.m., and finally to 0.2 p.p.m. As2O3. at the termination of the experiment on October 10, 1956. From July to October, the amount of plankton varied from 5.9 to 10.6 milligrams dry weight per liter. Arsenic content of the plankton from ponds treated with sodium arsenite increased gradually, and after 30 days ranged between 3,690 and 7,200 p.p.m. As203 of the dry weight; 76 days after treatment the arsenic content ranged between 1,636 and 3,600 p.p.m. As203. The arsenic content in the top inch of bottom soils from arsenic-treated ponds increased gradually, and at the termination of the experiment ranged between 14 and 54 p.p.m. AS203. As the arsenic in the soil increased, the percentage of arsenic that could be removed by leaching with distilled water also increased. Fish from each pond were analyzed for arsenic when the concentration in the water had declined to less than 1.0 p.p.m. As203. While small amounts of arsenic were found in the digestive tracts of some of the fish, little or no arsenic was detected in the flesh.