The U.S. Environmental Protection Agency is currently preparing recommendations for the Freshwater Quality Criteria for the compound Bis(tributyltin) Oxide (TBTO). As part of the supporting information for that recommendation, the USEPA has sited a study by Brooke at al. (1986) which reported that the most sensitive freshwater species to TBTO is the genus Hydra sp. with a EC-50 Value of 0.5 ug-1-1. This study was undertaken to investigate the sensitivity of several species of Hydra to TBTO and to fully understand the acute toxicity response of the organisms to this compound. The coelenterate Hydra sp. is a sensitive Freshwater organism found in a variety of lotic and lentic habitats. The species are found within the United States are adapted to a variety of habitats. The swift-water Hydra, Hydra littoralis is the species characteristic of many stream and spillway habitats, especially through the eastern states (Hyman, 1959). The green Hydra, Chlorohydra viridissina, is a small hydra with symbiotic zoochlorellae that is commonly found throughout the United States (Hyman, 1959).