Studies on effect of bis(tri-n-butyltin)oxide (TBTO) and other organotins on marine species have been conducted at the U.S. Environmental Protection Agency's laboratory at Gulf Breeze, Florida, since 1983. First studies were done on two species of algae, Skeletonema costatum and Thalassiosira pseudonana, where 72h EC50s for tributyltins and population growth were 0.35 and 1.16 microgram/l, respectively. Two developmental stages of the lugworm, Arenicola cristata, were sensitive to TBTO (96h LC50=4 microgram/l). Only 0.1 microgram/l inhibited arm regeneration by the brittle star, Ophioderma brevispina. TBTO was less toxic to the grass shrimp, Palaemonetes pugio, (96h LC50 = 20 microgram/l). Continuing studies include research on effects of TBTO on 1-, 4-and 10-day old mysids and estuarine seagrass communities.