The Registry of Tumors in Lower Animals (RTLA) of the National Museum of Natural History, Smithsonian Institution, Washington, DC is described. The general purpose of the RTLA is to facilitate the study of neoplasms in invertebrates and cold blooded vertebrates. The registry contains 3,064 specimens. Specimens are added to the registry at an annual rate of 324. The RTLA has seven main activities: to provide a diagnostic service, maintain a specimen depository, maintain a collection of pertinent literature, abstract and computerize the specimen and literature files, act as a clearinghouse for information, collaborate on field and experimental studies, and publicize the registry by means of published papers, lectures, personal contacts, and reports. The use of fish in carcinogenesis bioassays is discussed. There has been an increasing interest shown in using fish in environmental carcinogen surveys in recent years. Advantages of using fish in carcinogenesis bioassays include economy, efficient processing and evaluation, low genetic bias, high sensitivity, and the fact that many systems (such as the digestive, integumentary, nervous, and musculature) respond.