A single hierarchy of regulatory genes is responsible for specifying all of the somatic sexual differences between the two sexes in Drosophila melanogaster. This hierarchy has been well characterized both genetically and molecularly. Our current work is focused on how the proteins encoded by the final regulatory genes in the hierarchy function to bring about sex-specific patterns of gene activity, growth, morphogenesis, differentiation, and behavior. I will focus on recent work elucidating the genetic basis of the innate complex courtship behaviors of Drosophila males. One branch of the sex hierarchy, which contains the fruitless gene, is specifically responsible for male courtship behavior. Developmental and genetic studies have led to our working hypothesis that (1) fruitless functions during development to build the potential for male courtship behavior into the CNS and (2) that the fruitless expressing cells in the CNS comprise a circuit that subserves male courtship behavior. Our current studies of male courtship behavior address at the genetic, molecular and behavioral levels: (1) How is potential for male courtship behavior is established in the central nervous system (CNS) by the functioning of fruitless? and (2) How do the individual groups of fruitless expressing cells function together in the adult to insure the ordered manifestation of the events comprising this behavior.