N-nitroso-dimethylanine dissolved in aquarium water at a concentration of 400 ppm (1/4 of the LD50/30d) induced malignant tumors in 39 (54.2%) out of 72 young adult animals of the amphibian species Xenopus borealis. The first tumor was detected at 11 weeks, and the average latency was 35.1 weeks. Tumors were located in the liver (32 cholangiocarcinomas and 13 hepatocellular cancers) and the kidney (9 adenocarcinomas and 2 anaplastic cancers). Thirteen animals bore two or more types of tumors. These results demonstrate that a high percentage of tumors can be induced in amphibians by chemical carcinogens if appropriate methods are used.