Significant progress in the understanding of the neuroanatomical and chemical basis of Drosophila courtship behavior has recently been obtained by means of genetic techniques. Progress in understanding the changes in the courtship behavior in response to the genetically modified central nervous system was, however, hampered by the lack of an appropriate methodology for the measurement of behavior. Here, we propose an operational definition of behavior that provides a sensitive quantification of the differences among Drosophila courtship behaviors. Using this approach, we gain evidence that a highly specific and expressive body language underlies Drosophila courtship, offering the largest individual expression bandwidth for normal males courting mature females. We also find that normal male Drosophila perform a switch from male to female behavior if they are together with males carrying the fruitless gene. From this we conjecture that the courtship behavior of normal Drosophila is based upon a sensor-driven bistable neurodynamical system, whereas the ruitless mutations lack this adaptive ability. We anticipate our assay and the developed methodological approach to be a starting point for systematic investigations of the relationship between genes and behavior.