Benzimidazoles represent the only class of truly broad-spectrum anthelmintics, however, they also show activity against fungi and mammalian cells. This raises the question as to why benzimidazoles can selectively kill helminths and yet exhibit little or no mammalian toxicity. In this paper, Ernest Lacey examines this example of selectivity of drug action to the ubiquitous target of these drugs, the structural protein, tubulin.