Theperiodgene inDrosophila melanogastercontrols not only daily rhythms associated with adult emergence and behavior, but also a much higher frequency rhythm that accompanies the male's courtship song. This oscillation in the rate of sound production (normal period, ca. one minute) is either sped up (byper5), slowed down, or eliminated in the three classicpermutants. We have conducted a mosaic analysis in which both lovesong and cir-cadian locomotor cycles were examined in a series of flies that were each partper5and partper+.Consistent with previous studies, the focus forpercontrol of the adult's circadian rhythm of locomotion was found to be in the brain. However, several mosaic individuals were found to exhibit a mutant locomotor rhythm but a wild-type song cycle, or vice-versa, enabling us provisionally to map the song-rhythm focus to the thoracic ganglia. Thatperis expressed only in glial cells in the thoracic nervous system and, in general, mediates slow (hour-by-hour) fluctuations in the levels of its own products are discussed from the standpoint of the current mosaic mapping results and the renewed focus they bring to the gene's influence on an ultradian rhythm.