Internal clocks drive rhythms in behaviour and physiology in all eukaryotic organisms. The period (per) gene of the fruit fly Drosophila melanogaster controls the period of a ∼24hr locomotor activity rhythm and a ~55 second cycle in the male courtship song. D. melanogaster is one of eight very closely related species which comprise the melanogaster subgroup of Drosophila. The locomotor activity behaviour and courtship songs of several species from the melanogaster subgroup were analysed. Drosophila yakuba was found to have a cycle period of 70-80 seconds in its courtship song - longer than either D. melanogaster (50-60 seconds) or D. simulans (30-40 seconds), two previously characterised members of the subgroup. The period of the locomotor activity rhythms of four members of the subgroup, D. yakuba, D. melanogaster, D. erecta and D. teissieri, were indistinguishable from each other. However, each species exhibited unique patterns of activity within each daily cycle. The D. yakuba per locus was cloned, using the D. melanogaster per gene as a probe. The DNA sequences of two D. yakuba genes were obtained, per itself and a small adjacent gene. A region of the per protein which has been implicated in the control of the song cycle period, a repeating series of threonine glycine pairs, is shorter than in D. melanogaster. A patchy distribution of amino acid replacements was found between the D. melanogaster and D. yakuba per proteins, probably due to differences in selective constraints in different regions of the protein. In an effort to determine whether per determines species-specific aspects of behavioural rhythms, attempts were made to transduce mixed species per gene constructs into arrhythmic (per mutant) strains of D. melanogaster. These attempts were unsuccessful.