The courtship behaviour of both male and female Drosophila melanogaster changes with time. The durations and probabilities of occurrence of the various acts are affected. Some of the behaviour patterns show cycles in their bout durations. The patterns of change of the male and female behaviour show little evidence of interaction between the sexes. All the measured aspects of courtship song change significantly with time. Detailed analysis of the behavioural sequences during courtship indicates that some female acts have significant effects on the male"s behaviour and vice versa. Transitions in the behaviour of one sex cause a general decrease in the duration of the ongoing behaviour and a general decline in the probability of starting a new bout of behaviour in the other sex. Principal component analysis and canonical correlation analysis supported the conclusion that although there is significant male-female interaction these interactions do not have. large effects on the behaviour of the flies. A unitary motivational model for the courtship behaviour of the male was not supported. At least three underlying variables are required to explain the variation in the male's behaviour. The courtship behaviour of male D. melanogaster is highly determined. From a knowledge of the male's and female's behaviour from the start of the observation period it is possible to significantly predict the future behaviour of the male and the remaining time till copulation.