The question of how genes contribute to normal individual differences in behaviour has captured our imagination for more than a century. In the past, it was erroneously believed that genes and the environment acted exclusively of one another in determining behaviour. Currently, our ideas about how genes influence behaviour must incorporate the probabilistic and plastic nature of individual differences in behaviour. Accordingly, it could be hypothesized that different alleles (forms of the same gene) might give the animal different predispositions or probabilities of behaving in a certain manner in a given environment. This raises two fundamental questions: How do genes and their proteins act in the nervous system in order to cause normal individual differences in behaviour? How do genes and their proteins act in response to the environment to affect normal individual differences in behaviour? In my laboratory we study the genetic, molecular, neurobiological and environmental underpinnings of behavioural variation in the fruit fly Drosophila melanogaster. We study the mechanistic and evolutionary significance of genes that affect larval behaviour by isolating, identifying, cloning, and sequencing these genes and also by understanding how variation in gene expression can affect fitness of the organism. Gene discovery in Drosophila has proven useful for understanding how homologous genes function in mammals.