The development of the nematode C. elegans is highly invariant, and has been described in great detail. Many developmental mutations have been isolated and analysed; some of these identify switch genes, i.e. genes that control the choice between two developmental alternatives. It appears that the genetic controls of development in this animal are discrete, hierarchical and relatively simple. The control of sexual dimorphism provides an example of how a series of switch genes are organized in a regulatory cascade.