Large scale pollution of the environment by pesticides and other chemicals only started in the fifties and early sixties. Chlorinated hydrocarbon pesticides and methyl mercury compounds appeared to have a marked impact on the populations of many animal species, especially birds. It is recognized that prof. dr. H. van Genderen, who retired this month, made a most valuable contribution to the timely detection of these problems in the Netherlands. Environmental toxicologists engaged in studies to predict environmental hazards by chemicals require toxicological skill as well as insight into the structure and functioning of ecosystems.