A look at the Dutch Elm disease and biological efforts to control it. Host John Livingston outlines the history of the disease in Canada; explains the nature of the disease; how it is transmitted; the failure of attempts to stop it with DDT spraying; and methods of elm tree "sanitation". The technique of bilogical control and its dangers is examined including: the disastrous results of the introduction of the Indian mongoose in Trinidad to control rats; the biological control of rose aphids; and the successful campaign in Florida to eliminate the screw worm fly by the introduction of sterile males.