Measurements of arsenic concentrations in macrophytes, sediments, fish, and water were made in Lake Rotoroa (37°48'S, 175°16'30"E) and nearby Lake Rotokauri in April and June 1983. Lake Rotoroa, which had been sprayed with sodium arsenite in 1959 to control problem growths of aquatic weeds, showed elevated concentrations of As in its macrophytes (193–1200 mg (kg dry wt)−1) and surficial sediments (540–780 mg (kg dry wt)−1) compared to lake Rotokauri (< 20 and 16.5–40 mg (kg dry wt)−1, respectively). The levels of As recorded in the macrophytes and sediments of Lake Rotoroa were between one and two orders of magnitude greater than those reported from studies of uncontaminated lakes and in the high range of those recorded for lakes subject to geothermal influences or inputs from mining and industry. Sediment As concentrations in Lake Rotoroa were also greater than values reported for seven North American lakes subject to multiple applications of sodium arsenite herbicides. All fish tissues sampled had As concentrations below 1 mg (kg wet wt)−1suggesting minimal accumulation, and levels in the lakes waters were below 0.01 g m−3.