A new clock mutant, namedAndante, has been identified on the X chromosome ofDrosophila melanogaster. Andantelengthens the period of the circadian eclosion and locomotor activity rhythms by 1.5–2.0 hours. The phase response curves for the eclosion and activity rhythms, indicating light-induced phase shifts, show a similar degree of lengthening.Andantealso lengthens the periods of other clock mutants, includingClock, and alleles of theperiodlocus. Analysis of locomotor activity rhythms reveals thatAndanteis semi-dominant, andAndanterhythms are highly temperature compensated. Thesine oculismutation, which eliminates the outer visual system, has no effect on the period ofAndante.Deficiency mapping indicates thatAndanteis located in the IOEI-2 to IOFI region of the X chromosome, close to theminiature-duskylocus. WhereasAndanteflies have aduskywing phenotype,duskyflies do not have anAndanteclock phenotype.